diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/rabbit_variable_queue.erl | 25 |
1 files changed, 19 insertions, 6 deletions
diff --git a/src/rabbit_variable_queue.erl b/src/rabbit_variable_queue.erl index 094db17f4b..fc7624478a 100644 --- a/src/rabbit_variable_queue.erl +++ b/src/rabbit_variable_queue.erl @@ -31,13 +31,31 @@ -export([start_msg_store/2, stop_msg_store/0, init/6]). %%---------------------------------------------------------------------------- +%% Messages, and their position in the queue, can be in memory or on +%% disk, or both. Persistent messages will have both message and +%% position pushed to disk as soon as they arrive; transient messages +%% can be written to disk (and thus both types can be evicted from +%% memory) under memory pressure. The question of whether a message is +%% in RAM and whether it is persistent are orthogonal. +%% +%% Messages are persisted using the queue index and the message +%% store. Normally the queue index holds the position of the message +%% *within this queue* along with a couple of small bits of metadata, +%% while the message store holds the message itself (including headers +%% and other properties). +%% +%% However, as an optimisation, small messages can be embedded +%% directly in the queue index and bypass the message store +%% altogether. +%% %% Definitions: %% %% alpha: this is a message where both the message itself, and its %% position within the queue are held in RAM %% %% beta: this is a message where the message itself is only held on -%% disk, but its position within the queue is held in RAM. +%% disk (if persisted to the message store) but its position +%% within the queue is held in RAM. %% %% gamma: this is a message where the message itself is only held on %% disk, but its position is both in RAM and on disk. @@ -54,11 +72,6 @@ %% appears. It's frequently the case that gammas are defined by betas %% who have had their queue position recorded on disk. %% -%% Furthermore note that for small messages we can persist the message -%% to the queue index rather than the message store; if this happens -%% then the message will still be in memory when the message is a -%% beta. -%% %% In general, messages move q1 -> q2 -> delta -> q3 -> q4, though %% many of these steps are frequently skipped. q1 and q4 only hold %% alphas, q2 and q3 hold both betas and gammas. When a message |