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https://issues.apache.org/jira/browse/JAMES-3816?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Benoit Tellier closed JAMES-3816.
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Resolution: Fixed
> Limit IMAP concurrent requests
> ------------------------------
>
> Key: JAMES-3816
> URL: https://issues.apache.org/jira/browse/JAMES-3816
> Project: James Server
> Issue Type: Improvement
> Components: IMAPServer
> Reporter: Benoit Tellier
> Priority: Major
> Time Spent: 1h 20m
> Remaining Estimate: 0h
>
> h2. Context
> In James 3.7.0 IMAP concurrency model was based on the thread pool model: a
> pool of (eg) 20
> threads conducts 20 blocking tasks simultaneously.
> This lead to resource usage inefficiency: to handle 1000 requests at a given
> time, we need 1000 threads
> and OS threads are expensive.
> However, this also put some concurrency control in place that protects the
> IMAP server of bursts as no
> more requests than the count of threads could be processed.
> In JAMES-3737 we migrated to a reactive model. IMAP stack had been
> re-implemented to be asynchronous. A
> few threads (typically matching the count of CPU) would carry out an
> arbitrarily large amount of IMAP
> requests simultaneously. This is efficient resource wise, but there is no
> more some concurrency control
> mechanisms in place.
> On a production environment, I notice some bursts of a few seconds where an
> Apple mail client fires thousands
> of IMAP requests, which exhaust server resources: S3 connections, Cassandra
> queries times out at consistency serial.
> h2. Solution
> Put back concurrency control on top of the IMAP reactive stack.
> We can take inspiration from the Cassandra driver throttlers to do so:
> * Defines a maximum number of concurrent requests
> * Have a queue for request delayed because maximum execution level is exceeded
> * Once executed, execution of other requests are triggered
> * If exceeding the queue size requests are rejected
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