| = 23. Cassandra Mailbox Counters inconsistencies |
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| Date: 2020-03-07 |
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| == Status |
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| Accepted (lazy consensus) |
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| == Context |
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| Cassandra maintains a per mailbox projection for message count and unseen message count. |
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| As with any projection, it can go out of sync, leading to inconsistent results being returned to the client, which is not acceptable. |
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| Here is the table organisation: |
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| * `mailbox` Lists the mailboxes |
| * `messageIdTable` Holds mailbox and flags for each message, lookup by mailbox ID + UID |
| * `imapUidTable` Holds mailbox and flags for each message, lookup by message ID and serves as a source of truth |
| * `mailboxCounters` Holds messages count and unseen message count for each mailbox. |
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| Failures during the denormalization process will lead to inconsistencies between the counts and the content of `imapUidTable` |
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| This can lead to the following user experience: |
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| * Invalid message count can be reported in the Mail User Agent (IMAP & JMAP) |
| * Invalid message unseen count can be reported in the Mail User Agent (IMAP & JMAP) |
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| == Decision |
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| Implement a webadmin exposed task to recompute mailbox counters. |
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| This endpoints will: |
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| * List existing mailboxes |
| * List their messages using `messageIdTable` |
| * Check them against their source of truth `imapUidTable` |
| * Compute mailbox counter values |
| * And reset the value of the counter if needed in `mailboxCounters` |
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| == Consequences |
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| This endpoint is subject to data races in the face of concurrent operations. |
| Concurrent increments & decrements will be ignored during a single mailbox processing. |
| However the source of truth is unaffected hence, upon rerunning the task, the result will be eventually correct. |
| To be noted that Cassandra counters can't be reset in an atomic manner anyway. |
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| We rely on the "listing messages by mailbox" projection (that we recheck). |
| Missing entries in there will be ignored until the given projection is healed (currently unsupported). |
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| We furthermore can piggy back a partial check of the message denormalization described in xref:0021-cassandra-acl-inconsistency.adoc[this ADR] upon counter recomputation (partial because we cannot detect missing entries in the "list messages in mailbox" denormalization table) |
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| == References |
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| * https://github.com/linagora/james-project/pull/3125[Plan for fixing Cassandra ACL inconsistencies] |
| * https://www.mail-archive.com/server-dev@james.apache.org/msg64432.html[General mailing list discussion about inconsistencies] |
| * https://issues.apache.org/jira/browse/JAMES-3105[JAMES-3105 Related JIRA] |
| * https://github.com/linagora/james-project/pull/3185[Pull Request: JAMES-3105 Corrective task for fixing mailbox counters] |
