doc/SPOE.txt

                -----------------------------------------------
                   Stream Processing Offload Engine (SPOE)
                                  Version 1.0
                          ( Last update: 2016-11-07 )
                -----------------------------------------------
                          Author : Christopher Faulet
                      Contact : cfaulet at haproxy dot com


SUMMARY
--------

  0.      Terms
  1.      Introduction
  2.      SPOE configuration
  2.1.        SPOE scope
  2.2.        "spoe-agent" section
  2.3.        "spoe-message" section
  2.4.        Example
  3.      SPOP specification
  3.1.        Data types
  3.2.        Frames
  3.2.1.          Frame capabilities
  3.2.2.          Frame types overview
  3.2.3.          Workflow
  3.2.4.          Frame: HAPROXY-HELLO
  3.2.5.          Frame: AGENT-HELLO
  3.2.6.          Frame: NOTIFY
  3.2.7.          Frame: ACK
  3.2.8.          Frame: HAPROXY-DISCONNECT
  3.2.9.          Frame: AGENT-DISCONNECT
  3.3.        Events & messages
  3.4.        Actions
  3.5.        Error & timeouts


0. Terms
---------

* SPOE : Stream Processing Offload Engine.

    A SPOE is a filter talking to servers managed ba a SPOA to offload the
    stream processing. An engine is attached to a proxy. A proxy can have
    several engine. Each engine is linked to an agent and only one.

* SPOA : Stream Processing Offload Agent.

    A SPOA is a service that will receive info from a SPOE to offload the
    stream processing. An agent manages several servers. It uses a backend to
    reference all of them. By extension, these servers can also be called
    agents.

* SPOP : Stream Processing Offload Protocol, used by SPOEs to talk to SPOA
         servers.

    This protocol is used by engines to talk to agents. It is an in-house
    binary protocol described in this documentation.


1. Introduction
----------------

SPOE is a feature introduced in HAProxy 1.7. It makes possible the
communication with external components to retrieve some info. The idea started
with the problems caused by most ldap libs not working fine in event-driven
systems (often at least the connect() is blocking). So, it is hard to properly
implement Single Sign On solution (SSO) in HAProxy. The SPOE will ease this
kind of processing, or we hope so.

Now, the aim of SPOE is to allow any kind of offloading on the streams. First
releases, besides being experimental, won't do lot of things. As we will see,
there are few handled events and even less actions supported. Actually, for
now, the SPOE can offload the processing before "tcp-request content",
"tcp-response content", "http-request" and "http-response" rules. And it only
supports variables definition. But, in spite of these limited features, we can
easily imagine to implement SSO solution, ip reputation or ip geolocation
services.


2. SPOE configuration
----------------------

Because SPOE is implemented as a filter, To use it, you must declare a "filter
spoe" line in a proxy section (frontend/backend/listen) :

  frontend my-front
      ...
      filter spoe [engine <name>] config <file>
      ...

The "config" parameter is mandatory. It specififies the SPOE configuration
file. The engine name is optional. It can be set to declare the scope to use in
the SPOE configuration. So it is possible to use the same SPOE configuration
for several engines. If no name is provided, the SPOE configuration must not
contain any scope directive.

We use a separate configuration file on purpose. By commenting SPOE filter
line, you completly disable the feature, including the parsing of sections
reserved to SPOE. This is also a way to keep the HAProxy configuration clean.

A SPOE configuration file must contains, at least, the SPOA configuration
("spoe-agent" section) and SPOE messages ("spoe-message" section) attached to
this agent. Unused messages (not reference in "spoe-agent" section) will be
ignored.

IMPORTANT : The configuration of a SPOE filter must be located in a dedicated
file. But the backend used by a SPOA must be declared in HAProxy configuration
file.

2.1. SPOE scope
-------------------------

If you specify an engine name on the SPOE filter line, then you need to define
scope in the SPOE configuration with the same name. You can have several SPOE
scope in the same file. In each scope, you must define one and only one
"spoe-agent" section to configure the SPOA linked to your SPOE and several
"spoe-message" sections to describe messages sent to servers mananger by your
SPOA.

A SPOE scope starts with this kind of line :

  [<name>]

where <name> is the same engine name specified on the SPOE filter line. The
scope ends when the file ends or when another scope is found.

  Example :
      [my-first-engine]
          spoe-agent my-agent
              ...
          spoe-message msg1
              ...
          spoe-message msg2
              ...

      [my-second-engine]
          ...

If no engine name is provided on the SPOE filter line, no SPOE scope must be
found in the SPOE configuration file. All the file is considered to be in the
same anonymous and implicit scope.

2.2. "spoe-agent" section
--------------------------

For each engine, you must define one and only one "spoe-agent" section. In this
section, you will declare SPOE messages and the backend you will use. You will
also set timeouts and options to customize your agent's behaviour.


spoe-agent <name>
  Create a new SPOA with the name <name>. It must have one and only one
  "spoe-agent" definition by SPOE scope.

  Arguments :
    <name>   is the name of the agent section.

  following keywords are supported :
    - messages
    - option var-prefix
    - timeout hello|idle|ack
    - use-backend


messages <msg-name> ...
  Declare the list of SPOE messages that an agent will handle.

  Arguments :
    <msg-name>   is the name of a SPOE message.

  Messages declared here must be found in the same engine scope, else an error
  is triggered during the configuration parsing. You can have many "messages"
  lines.

  See also: "spoe-message" section.


option var-prefix <prefix>
  Define the prefix used when variables are set by an agent.

  Arguments :

    <prefix>   is the prefix used to limit the scope of variables set by an
               agent.

  To avoid conflict with other variables defined by HAProxy, all variables
  names will be prefixed. By default, the "spoe-agent" name is used. This
  option can be used to customize it.

  The prefix will be added between the variable scope and its name, separated
  by a '.'. It may only contain characters 'a-z', 'A-Z', '0-9', '.' and '_', as
  for variables name. In HAProxy configuration, you need to use this prefix as
  a part of the variables name. For example, if an agent define the variable
  "myvar" in the "txn" scope, with the prefix "my_spoe_pfx", then you should
  use "txn.my_spoe_pfx.myvar" name in your HAProxy configuration.

  An agent will never set new variables at runtime. It can only set new value
  for existing ones.


timeout ack <timeout>
  Set the maximum time to wait for an agent to receive the acknowledgement to a
  NOTIFY frame.

  Arguments :
    <timeout>   is the timeout value specified in milliseconds by default, but
                can be in any other unit if the number is suffixed by the unit,
                as explained at the top of this document.


timeout hello <timeout>
  Set the maximum time to wait for an agent to receive the AGENT-HELLO frame.

  Arguments :
    <timeout>   is the timeout value specified in milliseconds by default, but
                can be in any other unit if the number is suffixed by the unit,
                as explained at the top of this document.

  This timeout is an applicative timeout. It differ from "timeout connect"
  defined on backends.


timeout idle <timeout>
  Set the maximum time to wait for an agent to close an idle connection.

  Arguments :
    <timeout>   is the timeout value specified in milliseconds by default, but
                can be in any other unit if the number is suffixed by the unit,
                as explained at the top of this document.


use-backend <backend>
  Specify the backend to use. It must be defined.

  Arguments :
    <backend>   is the name of a valid "backend" section.


2.3. "spoe-message" section
----------------------------

To offload the stream processing, SPOE will send messages with specific
information at a specific moment in the stream life and will wait for
corresponding replies to know what to do.


spoe-message <name>
  Create a new SPOE message with the name <name>.

  Arguments :
    <name>   is the name of the SPOE message.

  Here you define a message that can be referenced in a "spoe-agent"
  section. Following keywords are supported :
    - args
    - event

  See also: "spoe-agent" section.


args [name=]<sample> ...
  Define arguments passed into the SPOE message.

  Arguments :
    <sample>   is a sample expression.

  When the message is processed, if a sample expression is not available, it is
  set to NULL. Arguments are processed in their declaration order and added in
  the message in that order. It is possible to declare named arguements.

  For example:
    args frontend=fe_id src dst


event <name>
  Set the event that triggers sending of the message.

  Argument :
    <name>   is the event name.

  Supported events are:
    - on-client-session
    - on-server-connectiob
    - on-frontend-tcp-request
    - on-backend-tcp-request
    - on-tcp-response
    - on-frontend-http-request
    - on-backend-http-request
    - on-http-response

  See section 3.5 about Events.

2.4. Example
-------------

Here is a simple but complete example that sends client-ip address to a ip
reputation service. This service can set the variable "ip_score" which is an
integer between 0 and 100, indicating its reputation (100 means totally safe
and 0 a blacklisted IP with no doubt).

  ###
  ### HAProxy configuration
    frontend www
        mode http
        bind *:80

        filter spoe engine ip-reputation config spoe-ip-reputation.conf

        # Reject connection if the IP reputation is under 20
        tcp-request content reject if  { var(sess.iprep.ip_score) -m int lt 20 }

        default_backend http-servers

    backend http-servers
        mode http
        server http A.B.C.D:80

    backend iprep-servers
        mode tcp
        balance roundrobin

        timeout connect 5s # greater than hello timeout
        timeout server  3m # greater than idle timeout

        server iprep1 A1.B1.C1.D1:12345
        server iprep2 A2.B2.C2.D2:12345

  ####
  ### spoe-ip-reputation.conf
    [ip-reputation]

    spoe-agent iprep-agent
        messages get-ip-reputation

        option var-prefix iprep

        timeout hello   2s
        timeout ack     10ms
        timeout idle    2m

        use-backend iprep-servers

    spoe-message get-ip-reputation
        args ip=src
        event on-client-session


3. SPOP specification
----------------------

3.1. Data types
----------------

Here is the bytewise representation of typed data:

    TYPED-DATA    : <TYPE:4 bits><FLAGS:4 bits><DATA>

Supported types and their representation are:

    TYPE                       |  ID | DESCRIPTION
  -----------------------------+-----+----------------------------------
     NULL                      |  0  |  NULL   : <0>
     Boolean                   |  1  |  BOOL   : <1+FLAG>
     32bits signed integer     |  2  |  INT32  : <2><VALUE:varint>
     32bits unsigned integer   |  3  |  UINT32 : <3><VALUE:varint>
     64bits signed integer     |  4  |  INT64  : <4><VALUE:varint>
     32bits unsigned integer   |  5  |  UNIT64 : <5><VALUE:varint>
     IPV4                      |  6  |  IPV4   : <6><STRUCT IN_ADDR:4 bytes>
     IPV6                      |  7  |  IPV6   : <7><STRUCT IN_ADDR6:16 bytes>
     String                    |  8  |  STRING : <8><LENGTH:varint><BYTES>
     Binary                    |  9  |  BINARY : <9><LENGTH:varint><BYTES>
    10 -> 15  unused/reserved  |  -  |  -
  -----------------------------+-----+----------------------------------

Variable-length integer (varint) are encoded using Peers encoding:


       0  <= X < 240        : 1 byte  (7.875 bits)  [ XXXX XXXX ]
      240 <= X < 2288       : 2 bytes (11 bits)     [ 1111 XXXX ] [ 0XXX XXXX ]
     2288 <= X < 264432     : 3 bytes (18 bits)     [ 1111 XXXX ] [ 1XXX XXXX ]   [ 0XXX XXXX ]
   264432 <= X < 33818864   : 4 bytes (25 bits)     [ 1111 XXXX ] [ 1XXX XXXX ]*2 [ 0XXX XXXX ]
 33818864 <= X < 4328786160 : 5 bytes (32 bits)     [ 1111 XXXX ] [ 1XXX XXXX ]*3 [ 0XXX XXXX ]
 ...

For booleans, the value (true or false) is the first bit in the FLAGS
bitfield. if this bit is set to 0, then the boolean is evaluated as false,
otherwise, the boolean is evaluated as true.

3.2. Frames
------------

Exchange between HAProxy and agents are made using FRAME packets. All frames
must be prefixed with their size encoded on 4 bytes in network byte order:

    <FRAME-LENGTH:4 bytes> <FRAME>

A frame always starts with its type, on one byte, followed by metadata
containing flags, on 4 bytes and a two variable-length integer representing the
stream identifier and the frame identifier inside the stream:

    FRAME       : <FRAME-TYPE:1 byte> <METADATA> <FRAME-PAYLOAD>
    METADATA    : <FLAGS:4 bytes> <STREAM-ID:varint> <FRAME-ID:varint>

Then comes the frame payload. Depending on the frame type, the payload can be
of three types: a simple key/value list, a list of messages or a list of
actions.

    FRAME-PAYLOAD    : <LIST-OF-MESSAGES> | <LIST-OF-ACTIONS> | <KV-LIST>

    LIST-OF-MESSAGES : [ <MESSAGE-NAME> <NB-ARGS:1 byte> <KV-LIST> ... ]
    MESSAGE-NAME     : <STRING>

    LIST-OF-ACTIONS  : [ <ACTION-TYPE:1 byte> <NB-ARGS:1 byte> <ACTION-ARGS> ... ]
    ACTION-ARGS      : [ <TYPED-DATA>... ]

    KV-LIST          : [ <KV-NAME> <KV-VALUE> ... ]
    KV-NAME          : <STRING>
    KV-VALUE         : <TYPED-DATA>

    FLAGS :   0     1-31
            +---+-----------+
            | F|            |
            | I|  RESERVED  |
            | N|            |
            +--+------------+

    FIN: Indicates that this is the final payload fragment. The first fragment
         may also be the final fragment.

Frames cannot exceed a maximum size negociated between HAProxy and agents
during the HELLO handshake. Most of time, payload will be small enough to send
it in one frame. But when supported by the peer, it will be possible to
fragment huge payload on many frames. This ability is announced during the
HELLO handshake and it can be asynmetric (supported by agents but not by
HAProxy or the opposite). The following rules apply to fragmentation:

  * An unfragemnted payload consists of a single frame with the FIN bit set.

  * A fragemented payload consists of several frames with the FIN bit clear and
    terminated by a single frame with the FIN bit set. All these frames must
    share the same STREAM-ID and FRAME-ID. And, of course, the FRAME-TYPE must
    be the same.

Beside the support of fragmented payload by a peer, some payload must not be
fragmented. See below for details.

IMPORTANT : The maximum size supported by peers for a frame must be greater or
equal to 256 bytes.

3.2.1. Frame capabilities
--------------------------

Here are the list of official capabilities that HAProxy and agents can support:

  * fragmentation: This is the abaility for a peer to support fragmented
                   payload in received frames.

Unsupported or unknown capabilities are silently ignored, when possible.

3.2.2. Frame types overview
----------------------------

Here are types of frame supported by SPOE. Frames sent by HAProxy come first,
then frames sent by agents :

    TYPE                       |  ID | DESCRIPTION
  -----------------------------+-----+-------------------------------------
     HAPROXY-HELLO             |  1  |  Sent by HAProxy when it opens a
                               |     |  connection on an agent.
                               |     |
     HAPROXY-DISCONNECT        |  2  |  Sent by HAProxy when it want to close
                               |     |  the connection or in reply to an
                               |     |  AGENT-DISCONNECT frame
                               |     |
     NOTIFY                    |  3  |  Sent by HAProxy to pass information
                               |     |  to an agent
  -----------------------------+-----+-------------------------------------
     AGENT-HELLO               | 101 |  Reply to a HAPROXY-HELLO frame, when
                               |     |  the connection is established
                               |     |
     AGENT-DISCONNECT          | 102 |  Sent by an agent just before closing
                               |     |  the connection
                               |     |
     ACK                       | 103 |  Sent to acknowledge a NOTIFY frame
  -----------------------------+-----+-------------------------------------

Unknown frames may be silently skipped.

3.2.3. Workflow
----------------

  * Successful HELLO handshake:

    HAPROXY                       AGENT SRV
       |       HAPROXY-HELLO         |
       | --------------------------> |
       |                             |
       |        AGENT-HELLO          |
       | <-------------------------- |
       |                             |


  * Error encountered by agent during the HELLO handshake:

    HAPROXY                       AGENT SRV
       |       HAPROXY-HELLO         |
       | --------------------------> |
       |                             |
       |     DISCONNECT + close()    |
       | <-------------------------- |
       |                             |

  * Error encountered by HAProxy during the HELLO handshake:

    HAPROXY                       AGENT SRV
       |       HAPROXY-HELLO         |
       | --------------------------> |
       |                             |
       |        AGENT-HELLO          |
       | <-------------------------- |
       |                             |
       |         DISCONNECT          |
       | --------------------------> |
       |                             |
       |     DISCONNECT + close()    |
       | <-------------------------- |
       |                             |

  * Notify / Ack exchange:

    HAPROXY                       AGENT SRV
       |          NOTIFY             |
       | --------------------------> |
       |                             |
       |           ACK               |
       | <-------------------------- |
       |                             |

  * Connection closed by haproxy:

    HAPROXY                       AGENT SRV
       |         DISCONNECT          |
       | --------------------------> |
       |                             |
       |     DISCONNECT + close()    |
       | <-------------------------- |
       |                             |

  * Connection closed by agent:

    HAPROXY                       AGENT SRV
       |     DISCONNECT + close()    |
       | <-------------------------- |
       |                             |

3.2.4. Frame: HAPROXY-HELLO
----------------------------

This frame is the first one exchanged between HAProxy and an agent, when the
connection is established. The payload of this frame is a KV-LIST. It cannot be
fragmented. STREAM-ID and FRAME-ID are must be set 0.

Following items are mandatory in the KV-LIST:

  * "supported-versions"    <STRING>

    Last SPOP major versions supported by HAProxy. It is a comma-separated list
    of versions, following the format "Major.Minor". Spaces must be ignored, if
    any. When a major version is announced by HAProxy, it means it also support
    all previous minor versions.

    Example: "2.0, 1.5" means HAProxy supports SPOP 2.0 and 1.0 to 1.5

  * "max-frame-size"    <UINT32>

    This is the maximum size allowed for a frame. The HAPROXY-HELLO frame must
    be lower or equal to this value.

  * "capabilities"    <STRING>

    This a comma-separated list of capabilities supported by HAProxy. Spaces
    must be ignored, if any.

To finish the HELLO handshake, the agent must return an AGENT-HELLO frame with
its supported SPOP version, the lower value between its maximum size allowed
for a frame and the HAProxy one and capabilities it supports. If an error
occurs or if an incompatibility is detected with the agent configuration, an
AGENT-DISCONNECT frame must be returned.

3.2.5. Frame: AGENT-HELLO
--------------------------

This frame is sent in reply to a HAPROXY-HELLO frame to finish a HELLO
handshake. As for HAPROXY-HELLO frame, STREAM-ID and FRAME-ID are also set
0. The payload of this frame is a KV-LIST and it cannot be fragmented.

Following items are mandatory in the KV-LIST:

  * "version"    <STRING>

    This is the SPOP version the agent supports. It must follow the format
    "Major.Minor" and it must be lower or equal than one of major versions
    announced by HAProxy.

  * "max-frame-size"    <UINT32>

    This is the maximum size allowed for a frame. It must be lower or equal to
    the value in the HAPROXY-HELLO frame. This value will be used for all
    subsequent frames.

  * "capabilities"    <STRING>

    This a comma-separated list of capabilities supported by agent. Spaces must
    be ignored, if any.

At this time, if everything is ok for HAProxy (supported version and valid
max-frame-size value), the HELLO handshake is successfully completed. Else,
HAProxy sends a HAPROXY-DISCONNECT frame with the corresponding error.

3.2.6. Frame: NOTIFY
---------------------

Information are sent to the agents inside NOTIFY frames. These frames are
attached to a stream, so STREAM-ID and FRAME-ID must be set. The payload of
NOTIFY frames is a LIST-OF-MESSAGES and, if supported by agents, it can be
fragmented.

NOTIFY frames must be acknowledge by agents sending an ACK frame, repeating
right STREAM-ID and FRAME-ID.

3.2.7. Frame: ACK
------------------

ACK frames must be sent by agents to reply to NOTIFY frames. STREAM-ID and
FRAME-ID found in a NOTIFY frame must be reuse in the corresponding ACK
frame. The payload of ACK frames is a LIST-OF-ACTIONS and, if supported by
HAProxy, it can be fragmented.

3.2.8. Frame: HAPROXY-DISCONNECT
---------------------------------

If an error occurs, at anytime, from the HAProxy side, a HAPROXY-DISCONNECT
frame is sent with information describing the error. HAProxy will wait an
AGENT-DISCONNECT frame in reply. All other frames will be ignored. The agent
must then close the socket.

The payload of this frame is a KV-LIST. It cannot be fragmented. STREAM-ID and
FRAME-ID are must be set 0.

Following items are mandatory in the KV-LIST:

  * "status-code"    <UINT32>

    This is the code corresponding to the error.

  * "message"    <STRING>

    This is a textual message describing the error.

For more information about known errors, see section "Errors & timeouts"

3.2.9. Frame: AGENT-DISCONNECT
-------------------------------

If an error occurs, at anytime, from the agent size, a AGENT-DISCONNECT frame
is sent, with information desribing the error. such frame is also sent in reply
to a HAPROXY-DISCONNECT. The agent must close the socket just after sending
this frame.

The payload of this frame is a KV-LIST. It cannot be fragmented. STREAM-ID and
FRAME-ID are must be set 0.

Following items are mandatory in the KV-LIST:

  * "status-code"    <UINT32>

    This is the code corresponding to the error.

  * "message"    <STRING>

    This is a textual message describing the error.

For more information about known errors, see section "Errors & timeouts"

3.3. Events & Messages
-----------------------

Information about streams are sent in NOTIFY frames. You can specify which kind
of information to send by defining "spoe-message" sections in your SPOE
configuration file. for each "spoe-message" there will be a message in a NOTIFY
frame when the right event is triggered.

A NOTIFY frame is sent for an specific event when there is at least one
"spoe-message" attached to this event. All messages for an event will be added
in the same NOTIFY frame.

Here is the list of supported events:

  * on-client-session         is triggered when a new client session is created.
                              This event is only available for SPOE filters
                              declared in a frontend or a listen section.

  * on-frontend-tcp-request   is triggered just before the evaluation of
                              "tcp-request content" rules on the frontend side.
                              This event is only available for SPOE filters
                              declared in a frontend or a listen section.

  * on-backend-tcp-request    is triggered just before the evaluation of
                              "tcp-request content" rules on the backend side.
                              This event is skipped for SPOE filters declared
                              in a listen section.

  * on-frontend-http-request  is triggered just before the evaluation of
                              "http-request" rules on the frontend side. This
                              event is only available for SPOE filters declared
                              in a frontend or a listen section.

  * on-backend-http-request   is triggered just before the evaluation of
                              "http-request" rules on the backend side. This
                              event is skipped for SPOE filters declared in a
                              listen section.

  * on-server-session         is triggered when the session with the server is
                              established.

  * on-tcp-response           is triggered just before the evaluation of
                              "tcp-response content" rules.

  * on-http-response          is triggered just before the evaluation of
                              "http-response" rules.


The stream processing will loop on these events, when triggered, waiting the
agent reply.

3.4. Actions
-------------

An agent must acknowledge each NOTIFY frame by sending the corresponding ACK
frame. Actions can be added in these frames to dynamically take action on the
processing of a stream.

Here is the list of supported actions:

  * set-var    set the value for an existing variable. 3 arguments must be
               attached to this action: the variable scope (proc, sess, txn,
               req or req), the variable name (a string) and its value.

    ACTION-SET-VAR  : <SET-VAR:1 byte><NB-ARGS:1 byte><VAR-SCOPE:1 byte><VAR-NAME><VAR-VALUE>

    SET-VAR     : <1>
    NB-ARGS     : <3>
    VAR-SCOPE   : <PROCESS> | <SESSION> | <TRANSACTION> | <REQUEST> | <RESPONSE>
    VAR-NAME    : <STRING>
    VAR-VALUE   : <TYPED-DATA>

    PROCESS     : <0>
    SESSION     : <1>
    TRANSACTION : <2>
    REQUEST     : <3>
    RESERVED    : <4>

  * unset-var    unset the value for an existing variable. 2 arguments must be
                 attached to this action: the variable scope (proc, sess, txn,
                 req or req) and the variable name (a string).

    ACTION-UNSET-VAR  : <SET-VAR:1 byte><NB-ARGS:1 byte><VAR-SCOPE:1 byte><VAR-NAME>

    SET-VAR     : <1>
    NB-ARGS     : <3>
    VAR-SCOPE   : <PROCESS> | <SESSION> | <TRANSACTION> | <REQUEST> | <RESPONSE>
    VAR-NAME    : <STRING>

    PROCESS     : <0>
    SESSION     : <1>
    TRANSACTION : <2>
    REQUEST     : <3>
    RESERVED    : <4>


NOTE: Name of the variables will be automatically prefixed by HAProxy to avoid
      name clashes with other variables used in HAProxy. Moreover, unknown
      variable will be silently ignored.

3.5. Error & timeouts
----------------------

Here is the list of all known errors:

    STATUS CODE   |  DESCRIPTION
  ----------------+--------------------------------------------------------
     0            | normal (no error occurred)
     1            | I/O error
     2            | A timeout occurred
     3            | frame is too big
     4            | invalid frame received
     5            | version value not found
     6            | max-frame-size value not found
     7            | capabilities value not found
     8            | unsupported version
     9            | max-frame-size too big or too small
     99           | an unknown error occurrde
  ----------------+--------------------------------------------------------

An agent can define its own errors using a not yet assigned status code.

IMPORTANT NOTE: For a specific stream, when an abnormal/unexpected error
                occurs, the SPOE is disabled for all the transaction. So if you
                have several events configured, such error on an event will
                disabled all followings. For TCP streams, this will disable the
                SPOE for the whole session. For HTTP streams, this will disable
                it for the transaction (request and response).

To avoid a stream to wait infinitly, you must carefully choose the
acknowledgement timeout. In most of cases, it will be quiet low. But it depends
on the responsivness of your service.

You must also choose idle timeout carefully. Because connection with your
service depends on the backend configuration used by the SPOA, it is important
to use a lower value for idle timeout than the server timeout. Else the
connection will be closed by HAProxy. The same is true for hello timeout. You
should choose a lower value than the connect timeout.


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