The Cyrus IMAP Aggregator transparently distributes IMAP and POP mailboxes across multiple servers. Unlike other systems for load balancing IMAP mailboxes, the aggregator allows users to access mailboxes on any of the IMAP servers in the system.
The software described below is now available as part of the Cyrus IMAP distribution , versions 2.1.3 and higher. Please refer to the documentation for setup and install instructions.
The second approach has the benefit that there is no longer a single point of failure and the aggregate cost of multiple machines may be significantly lower than the cost of a single large machine. However, the system may be harder to implement as well as harder to manage.
In the IMAP space, the approach of buying a larger machine is pretty obvious. Distributing the load is a bit trickier since there is no concept of mailbox location in IMAP (excluding RFC2193 mailbox referrals, which are not widely implemented by clients). Clients traditionally assume that the server they are talking to is the server with the mailbox they are looking for.
The approaches to distributing the load among IMAP servers generally sacrifice the unified system image. For pure email, this is an acceptable compromise; however, trying to share mailboxes becomes difficult or even impossible. Specific examples can be found in Appendix A: DNS Name Load Balancing and Appendix B: IMAP Multiplexing).
We propose a new approach to overcome these problems. We call it the Cyrus IMAP Aggregator. The Cyrus aggregator takes a murder of IMAP servers and presents a server independent view to the clients. That is, all the mailboxes across all the IMAP servers are aggregated to a single image, thereby appearing to be only one IMAP server to the clients.
The Cyrus IMAP Aggregator requires version 2.1.3 or higher of the Cyrus IMAP Distribution.
The backend servers serve the actual data and are fully functional standalone IMAP servers that serve a set of mailboxes. Each backend server maintains a local mailboxes database that lists what mailboxes are available on that server.
The imapd processes on a backend server can stand by themselves, so that each backend IMAP server can be used in isolation, without a MUPDATE server or any frontend servers. However, they are configured so that they won't process any mailbox operations (CREATE, DELETE, RENAME, SETACL, etc) unless the master MUPDATE server can be contacted and authorizes the transaction.
In this mode, the imapd processes update the local mailboxes database themselves. Additionally, on a CREATE they need to reserve a place with the MUPDATE server to insure that other backend servers aren't creating the same mailbox before proceeding. Once the local aspects of mailbox creation are complete, the mailbox is activated on the MUPDATE server, and is considered available to any client through the frontends.
The front end servers, unlike the back end servers, are fully interchangeable with each other and the front end servers can be considered 'dataless'; any loss of a proxy results in no loss of data. The only persistent data that is needed (the mailbox list) is kept on the MUPDATE master server. This list is synchronized between the frontend and the MUPDATE master when the frontend comes up.
The list of mailboxes in the murder is maintained by the MUPDATE server. The MUPDATE protocol is described at RFC3656.
For IMAP service on a frontend, there are two main types of processes, the proxyd and the mupdate (slave mode) synchronization process. The proxyd handles the IMAP session with clients. It relies on a consistent and complete mailboxes database that reflects the state of the world. It never writes to the mailboxes database. Instead, the mailboxes database is kept in sync with the master by a slave mupdate process.
The incoming mail messages go to an lmtp proxy (either running on a frontend, a mail exchanger, or any other server). The lmtp proxy running on the front end server uses the master MUPDATE server to determine the location of the destination folder and then transfers the message to the appropriate back end server via LMTP. If the backend is not up (or otherwise fails to accept the message), then the LMTP proxy returns a failure to the connected MTA.
If a sieve script is present, the lmtp proxy server must do the processing as the end result of the processing may result in the mail message going to a different back end server than where the user's INBOX is. Note that the current implementation runs SIEVE on the backend servers, and holds the requirement that all of a user's mailboxes live on the same backend.
Clients that support RFC2193 IMAP referrals can bypass the aggregator front end. See section 3.8 for more details.
Clients are encouraged to bypass the front ends via approved mechanisms. This should result in better performance for the client and less load for the servers.
A001 CREATE INBOX.new
A002 SELECT INBOX.new
The user authenticates to the frontend server via any supported SASL mechanism or via plaintext. If authentication is successful, the front end server will authenticate to the back end server using a SASL mechanism (in our case GSSAPI) as a privileged user. This user is able to switch to the authorization of the actual user being proxied for and any authorization checks happen as if the user actually authenticated directly to the back end server. Note this is a native feature of many SASL mechanisms and nothing special with the aggregator.
To help protect the backends from a compromised frontends, all administrative actions (creating users, top level mailboxes, quota changes, etc) must be done directly from the client to the backend, as administrative permissions are not granted to any of the proxy servers. IMAP Referrals provide a way to accomplish this with minimal client UI changes.
If the user's INBOX server is down and the LSUB fails, then the aggregator replies with NO with an appropriate error message. Clients should not assume that the user has no subscriptions (though apparently some clients do this).
A quota root is not allowed to span across multiple servers. Atleast, not with the semantics that it will be inclusive across the murder.
[SELECT]
To SELECT a mailbox:
SELECT on mailboxes that do not exist are much more expensive but the assumption is that this does not frequently occur (or if it does, it is just after the mailbox has been created and the frontend hasn't seen the update yet).
COPY is somewhat special as it acts upon messages in the currently SELECT'd mailbox but then interacts with another mailbox.
In the case where the destination mailbox is on the same back end server as the source folder, the COPY command is issued to the back end server and the back end server takes care of the command.
If the destination folder is on a different back end server, the front end intervenes and does the COPY by FETCHing the messages from the source back end server and then APPENDs the messages to the destination server.
[LIST]
[CREATE]
CREATE creates the mailbox on the same back end server as the
parent mailbox. If the parent exists but exists on multiple back end
servers, if there is no parent folder, a tagged NO response is
returned.
When this happens, the administrator has two choices. He may connect directly to a back end server and issue the CREATE on that server. Alternatively, a second argument can be given to CREATE after the mailbox name. This argument specifies the specific host name on which the mailbox is to be created.
The following operations occur for CREATE on the front end:
The following operations occur for CREATE on the back end:
Failure modes: Above, all back end inconsistencies result in the next CREATE attempt failing. The earlier MUPDATE inconsistency results in any attempts to CREATE the mailbox on another back end failing. The latter one makes the mailbox unreachable and un-createable. Though, this is safer than potentially having the mailbox appear in two places when the failed backend comes back up.
[RENAME]
RENAME is only interesting in the cross-server case. In this
case it issues a (non-standard) XFER command to the backend that currently
hosts the mailbox, which performs a binary transfer of the mailbox (and in
the case of a user's inbox, their associated seen state and subscription
list) to the new backend. During this time the mailbox is marked as
RESERVED in mupdate, and when it is complete it is activated on the new
server in MUPDATE. The deactivation prevents clients from accessing the
mailbox, and causes mail delivery to temporarily fail.
If clients support IMAP Mailbox Referrals [MBOXREF], the client can improve performance and reduce the load on the aggregator by using the IMAP referrals that are sent to it and going to the appropriate back end servers.
The front end servers will advertise the MAILBOX-REFERRALS capability. The back end servers will also advertise this capability (but only because they need to refer clients while a mailbox is moving between servers).
Since there is no way for the server to know if a client supports referrals, the Cyrus IMAP Aggregator will assume the clients do not support referrals unless the client issues a RLSUB or a RLIST command.
Once a client issues one of those commands, then the aggregator will issue referrals for any command that is safe for the client to contact the IMAP server directly. Most commands that perform operations within a mailbox (cf Section 3.3) fall into this category. Some commands will not be possible without a referrals-capable client (such as most commands done as administrator).
RFC2193 indicates that the client does not stick the referred server. As such the SELECT will get issued to the front end server and not the referred server. Additionally, CREATE, RENAME, and DELETE get sent to the frontend which will proxy the command to the correct back end server.
The mupdate (slave) process (one per front end) holds open an MUPDATE connection and listens for updates from the MUPDATE master server (as backends inform it of updates). The slave makes these modifications on the local copy of the mailboxes database.
??? Add timing info? Random load testing?
A benefit of having the mailbox information on the front end is that LIST is very cheap. The front end servers can process this request without having to contact each back end server.
We're also assuming that LIST is a much more frequent operation than any of the mailbox operations and thus should be the case to optimize. (In addition to the fact that any operation that needs to be forwarded to a backend needs to know which backend it is being forwarded to, so lookups in the mailbox list are also quite frequent).
What happens when a back end server comes up? Resynchronization with the MUPDATE server. Any mailboxes that exist locally but are not in MUPDATE are pushed to MUPDATE. Any mailboxes that exist locally but are in MUPDATE as living on a different server are deleted. Any mailboxes that do not exist locally but exist in MUPDATE as living on this server are removed from MUPDATE.
What happens when a front end server comes up? The only thing that needs to happen is for the front end to connect to the MUPDATE server, issue an UPDATE command, and resynchronize its local database copy with the copy on the master server.
Where's the true mailboxes file? The MUPDATE master contains authoritative information as to the location of any mailbox (in the case of a conflict), but the backends are authoritative as to which mailboxes actually exist.
The failure of the back-ends will result in the loss of availability. However, given that the data is distributed among multiple servers, the failure of a single server does not result the entire system being down. Our experience with AFS was that this type of partitioned failure was acceptable (if not ideal).
The failure of the mupdate master will cause write operations to the mailbox list to fail, but accesses to mailboxes themselves (as well as read operations to the mailbox list) will continue uninterrupted.
At this point, there may be some ideas but no plans for providing a high availability solution which would allow for back-end servers or the MUPDATE server to fail with no availability impact.
Live statistics can be found under the "cyrus overview" section at: http://graphs.andrew.cmu.edu.
Users don't lose the ability to share their folders and those folders are visible to other users. A user's INBOX folder hierarchy can also exist across multiple machines.
One method of load balancing is to use DNS to spread your users to multiple machines.
One method is to create a DNS CNAME for each letter of the alphabet. Then, each user sets their IMAP server to be the first letter of their userid. For example, the userid 'tom' would set his IMAP server to be T.IMAP.ANDREW.CMU.EDU and T.IMAP.ANDREW.CMU.EDU would resolve to an actual mail server.
Given that this does not provide a good distribution, another option is to create a DNS CNAME for each user. Using the previous example, the user 'tom' would set his IMAP server to be TOM.IMAP.ANDREW.CMU.EDU which then points to an actual mail server.
The good part is that you don't have all your users on one machine and growth can be accommodated without any user reconfiguration.
The drawback is with shared folders. The mail client now must support multiple servers and users must potentially configure a server for each user with a shared folder he wishes to view. Also, the user's INBOX hierarchy must also reside on a single machine.
Another method of spreading out the load is to use IMAP multiplexing. This is very similar to the IMAP Aggregator in that there are frontend and backend servers. The frontend servers do the lookup and then forward the request to the appropriate backend server.
The multiplexor looks at the user who has authenticated. Once the user has authenticated, the frontend does a lookup for the backend server and then connects the session to a single backend server. This provides the flexibility of balancing the users among any arbitrary server but it creates a problem where a user can not share a folder with a user on a different back end server.
Multiplexors references:
[LSUB, SUBSCRIBE, UNSUBSCRIBE, SETQUOTA, GETQUOTA, EXAMINE,
STATUS]
All these commands could be handled by the mailboxes
dataset.[MBOXDSET] This would mainly be a client enhancement. For
example, a client could use this to quickly check all the new messages
in every folder a user is subscribed to and would also be able to talk
directly to the back-end where the data is and thereby skipping the
front-ends.
An alternate name was suggested (unfortunately too late) by Philip Lewis. He writes:
i would have called it an atlas.... or iatlas (pronounced "yatlas")
it is a collection of (i)maps.