Willy Tarreau | 172945f | 2019-08-08 15:28:52 +0200 | [diff] [blame] | 1 | /* |
| 2 | * include/types/ring.h |
| 3 | * This file provides definitions for ring buffers used for disposable data. |
| 4 | * |
| 5 | * Copyright (C) 2000-2019 Willy Tarreau - w@1wt.eu |
| 6 | * |
| 7 | * This library is free software; you can redistribute it and/or |
| 8 | * modify it under the terms of the GNU Lesser General Public |
| 9 | * License as published by the Free Software Foundation, version 2.1 |
| 10 | * exclusively. |
| 11 | * |
| 12 | * This library is distributed in the hope that it will be useful, |
| 13 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 14 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 15 | * Lesser General Public License for more details. |
| 16 | * |
| 17 | * You should have received a copy of the GNU Lesser General Public |
| 18 | * License along with this library; if not, write to the Free Software |
| 19 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA |
| 20 | */ |
| 21 | |
| 22 | #ifndef _TYPES_RING_H |
| 23 | #define _TYPES_RING_H |
| 24 | |
Willy Tarreau | a8561db | 2020-03-06 18:40:31 +0100 | [diff] [blame] | 25 | #include <common/buf.h> |
Willy Tarreau | 172945f | 2019-08-08 15:28:52 +0200 | [diff] [blame] | 26 | #include <common/compat.h> |
| 27 | #include <common/config.h> |
| 28 | #include <common/ist.h> |
| 29 | |
| 30 | /* The code below handles circular buffers with single-producer and multiple |
| 31 | * readers (up to 255). The buffer storage area must remain always allocated. |
| 32 | * It's made of series of payload blocks followed by a readers count (RC). |
| 33 | * There is always a readers count at the beginning of the buffer as well. Each |
| 34 | * payload block is composed of a varint-encoded size (VI) followed by the |
| 35 | * actual payload (PL). |
| 36 | * |
| 37 | * The readers count is encoded on a single byte. It indicates how many readers |
| 38 | * are still waiting at this position. The writer writes after the buffer's |
| 39 | * tail, which initially starts just past the first readers count. Then it |
| 40 | * knows by reading this count that it must wake up the readers to indicate |
| 41 | * data availability. When a reader reads the payload block, it increments the |
| 42 | * next readers count and decrements the current one. The area between the |
| 43 | * initial readers count and the next one is protected from overwriting for as |
| 44 | * long as the initial count is non-null. As such these readers count are |
| 45 | * effective barriers against data recycling. |
| 46 | * |
| 47 | * Only the writer is allowed to update the buffer's tail/head. This ensures |
| 48 | * that events can remain as long as possible so that late readers can get the |
| 49 | * maximum history available. It also helps dealing with multi-thread accesses |
| 50 | * using a simple RW lock during the buffer head's manipulation. The writer |
| 51 | * will have to delete some old records starting at the head until the new |
| 52 | * message can fit or a non-null readers count is encountered. If a message |
| 53 | * cannot fit due to insufficient room, the message is lost and the drop |
| 54 | * counted must be incremented. |
| 55 | * |
| 56 | * Like any buffer, this buffer naturally wraps at the end and continues at the |
| 57 | * beginning. The creation process consists in immediately adding a null |
| 58 | * readers count byte into the buffer. The write process consists in always |
| 59 | * writing a payload block followed by a new readers count. The delete process |
| 60 | * consists in removing a null readers count and payload block. As such, there |
| 61 | * is always at least one readers count byte in the buffer available at the |
| 62 | * head for new readers to attach to, and one before the tail, both of which |
| 63 | * may be the same when the buffer doesn't contain any event. It is thus safe |
| 64 | * for any reader to simply keep the absolute offset of the last visited |
| 65 | * position and to restart from there. The write will update the buffer's |
| 66 | * absolute offset when deleting entries. All this also has the benefit of |
| 67 | * allowing a buffer to be hot-resized without losing its contents. |
| 68 | * |
| 69 | * Thus we have this : |
| 70 | * - init of empty buffer: |
| 71 | * head-, ,-tail |
| 72 | * [ RC | xxxxxxxxxxxxxxxxxxxxxxxxxx ] |
| 73 | * |
| 74 | * - reader attached: |
| 75 | * head-, ,-tail |
| 76 | * [ RC | xxxxxxxxxxxxxxxxxxxxxxxxxx ] |
| 77 | * ^- +1 |
| 78 | * |
| 79 | * - append of one event: |
| 80 | * appended |
| 81 | * head-, <----------> ,-tail |
| 82 | * [ RC | VI | PL | RC | xxxxxxxxxxx ] |
| 83 | * |
| 84 | * - reader advancing: |
| 85 | * head-, ,-tail |
| 86 | * [ RC | VI | PL | RC | xxxxxxxxxxx ] |
| 87 | * ^- -1 ^- +1 |
| 88 | * |
| 89 | * - writer removing older message: |
| 90 | * head-, ,-tail |
| 91 | * [ xxxxxxxxxxxx | RC | xxxxxxxxxxx ] |
| 92 | * <----------> |
| 93 | * removed |
| 94 | */ |
| 95 | |
| 96 | struct ring { |
| 97 | struct buffer buf; // storage area |
| 98 | size_t ofs; // absolute offset in history of the buffer's head |
Willy Tarreau | 1d181e4 | 2019-08-30 11:17:01 +0200 | [diff] [blame] | 99 | struct list waiters; // list of waiters, for now, CLI "show event" |
Willy Tarreau | 172945f | 2019-08-08 15:28:52 +0200 | [diff] [blame] | 100 | __decl_hathreads(HA_RWLOCK_T lock); |
| 101 | int readers_count; |
| 102 | }; |
| 103 | |
| 104 | #endif /* _TYPES_RING_H */ |
| 105 | |
| 106 | /* |
| 107 | * Local variables: |
| 108 | * c-indent-level: 8 |
| 109 | * c-basic-offset: 8 |
| 110 | * End: |
| 111 | */ |