Cyrus Murder: Concepts
**********************

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.

Note that although this document focuses on IMAP and POP, the same
concepts also apply to NNTP and HTTP (CalDAV, CardDAV, RSS).


Overview
========

Scaling a service usually takes one of two paths:

1. Buy bigger and faster machines

      This approach is obvious and (hopefully) easy, though at some
      point software tuning becomes necessary to take advantage of the
      bigger machines. However, if one of these large machines go
      down, then your entire system is unavailable.

2. Distribute the load across multiple machines.

      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 **RFC 2193** 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.


Architecture
============

The Cyrus IMAP Aggregator has three classes of servers:
   1. IMAP frontend,

   2. IMAP backend, and

   3. MUPDATE.

The frontend servers act as the primary communication point between
the end user clients and the backend servers. The frontends use the
MUPDATE server as an authoritative source for mailbox names,
locations, and permissions. The backend servers store the actual IMAP
data (and keep the MUPDATE server appraised as to changes in the
Mailbox list).


Backend Servers
---------------

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.


Frontend Servers
----------------

The frontend servers, unlike the backend servers, are fully
interchangeable with each other and the frontend 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 **RFC 3656**.

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.


Mail Delivery
-------------

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 frontend server uses the master MUPDATE server to
determine the location of the destination folder and then transfers
the message to the appropriate backend 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 backend server than where the user’s
INBOX is.

Note: 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
-------

Clients that support **RFC 2193** IMAP referrals can bypass the
aggregator frontend. See IMAP Referrals for more details.

Clients are encouraged to bypass the frontends via approved
mechanisms. This should result in better performance for the client
and less load for the servers.

Note: Sites choosing to locate front ends in a DMZ, or other
  isolated network segment, should disable IMAP Referrals which might
  lead clients to attempt impossible actions, such as direct access to
  back ends which are firewalled. Please consult imapd.conf(5) for
  details of the "proxyd_disable_mailbox_referrals" setting.


Implementation
==============


Assumptions
-----------

* Operations that change the mailbox list are (comparatively) rare.
  The vast majority of IMAP sessions do not manipulate the state of
  the mailbox list.

* Read operations on the mailbox list are very frequent.

* A mailbox name must be unique among all the back end servers.

* The MUPDATE master server will be able to handle the load from the
  frontend, backend, and LMTP proxy servers. Currently, the MUPDATE
  master can be a bottleneck in the throughput of mailbox operations,
  but as the MUPDATE protocol allows for slave server to act as
  replicas, it is theoretically possible to reduce the load of read
  operations against the master to a very low level.

* IMAP clients are not sensitive to somewhat loose mailbox tree
  consistency, and some amount of consistency can be sacrificed for
  speed. As is, IMAP gives no guarantees about the state of the
  mailbox tree from one command to the next. However, it’s important
  to note that different IMAP sessions do communicate out of band: two
  sessions for the same client should see sensible results. In the
  Murder case, this means that the same client talking to two
  different frontends should see sensible results.

* A single IMAP connection should see consistent results: once an
  operation is done, it is done, and needs to be reflected in the
  current session. The straightforward case that must work correctly
  is (provided there is no interleaved DELETE in another session):

     A001 CREATE INBOX.new
     A002 SELECT INBOX.new

* Accesses to non-existent mailboxes are rare.


Authentication
--------------

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 backend server using a SASL
mechanism (in our case KERBEROS_V4 or 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 backend 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.


Subscriptions
-------------

"[LSUB, SUBSCRIBE, UNSUBSCRIBE]"

The frontend server directs the LSUB to the backend server that has
the user’s INBOX. As such, the backend server may have entries in the
subscription database that do not exist on that server. The frontend
server needs to process the list returned by the backend server and
either remove or tag with \NoSelect the entries which are not
currently active within the murder.

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).


Finding a Mailbox
-----------------

"[SETQUOTA, GETQUOTA, EXAMINE, STATUS]"

The frontend machine looks up the location of the mailbox, connects
via IMAP to the backend server, and issues the equivalent command
there. A quota root is not allowed to span across multiple servers. At
least, not with the semantics that it will be inclusive across the
murder.

"[SELECT]"

   To SELECT a mailbox:

   1. proxyd: lookup foo.bar in local mailboxes database

   2. if yes, proxyd -> back end: send SELECT

   3. if no, proxyd -> mupdate slave -> mupdate master: send a ping
      along the UPDATE channel in order to ensure that we have
      received the latest data from the MUPDATE master.

   4. if mailbox still doesn’t exist, fail operation

   5. if mailbox does exist, and the client supports referrals,
      refer the client. Otherwise continue as a proxy with a selected
      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).


Operations within a Mailbox
---------------------------

"[APPEND, CHECK, CLOSE, EXPUNGE, SEARCH, FETCH, STORE, UID]"

These commands are sent to the appropriate backend server. The
aggregator does not need to modify any of these commands before
sending them to the backend server.


COPY
----

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 backend
server as the source folder, the COPY command is issued to the backend
server and the backend server takes care of the command.

If the destination folder is on a different backend server, the
frontend intervenes and does the COPY by FETCHing the messages from
the source backend server and then APPENDs the messages to the
destination server.


Operations on the Mailbox List
------------------------------

"[CREATE, DELETE, RENAME, SETACL]"

   These commands are all done by the backend server using the MUPDATE
   server as a lock manager. Changes are then propagated to the
   frontend via the MUPDATE protocol.

"[LIST]"

   LIST is handled by the frontend servers; no interaction is required
   with the backend server as the front ends have a local database
   that is never more than a few seconds out of date.

"[CREATE]"

   CREATE creates the mailbox on the same backend server as the parent
   mailbox. If the parent exists but exists on multiple backend
   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 backend 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 frontend:

   * proxyd: verify that mailbox doesn’t exist in MUPDATE mailbox
     list.

   * proxyd: decide where to send CREATE (the server of the parent
     mailbox, as top level mailboxes cannot be created by the
     proxies).

   * proxyd -> back end: duplicate CREATE command and verifies that
     the CREATE does not create an inconsistency in the mailbox list
     (i.e. the folder name is still unique).

   The following operations occur for CREATE on the backend:

   * imapd: verify ACLs to best of ability (CRASH: aborted)

   * imapd: start mailboxes transaction (CRASH: aborted)

   * imapd may have to open an MUPDATE connection here if one
     doesn’t already exist

   * imapd -> MUPDATE: set foo.bar reserved (CRASH: MUPDATE
     externally inconsistent)

   * imapd: create foo.bar in spool disk (CRASH: MUPDATE externally
     inconsistent, back end externally inconsistent, this can be
     resolved when the backend comes back up by clearing the state
     from both MUPDATE and the backend)

   * imapd: add foo.bar to mailboxes dataset (CRASH: ditto)

   * imapd: commit transaction (CRASH: ditto, but the recovery can
     activate the mailbox in mupdate instead)

   * imapd -> MUPDATE: set foo.bar active (CRASH: committed)

   Failure modes: Above, all backend inconsistencies result in the
   next CREATE attempt failing. The earlier MUPDATE inconsistency
   results in any attempts to CREATE the mailbox on another backend
   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.


IMAP Referrals
--------------

If clients support IMAP Mailbox Referrals (**RFC 2193**), 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
backend servers.

The frontend servers will advertise the "MAILBOX-REFERRALS"
capability. The backend 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 for which the client may safely
contact the IMAP server directly. Most commands performing 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).

**RFC 2193** indicates that the client does not stick the referred
server. As such the SELECT will get issued to the frontend server and
not the referred server. Additionally, CREATE, RENAME, and DELETE get
sent to the frontend which will proxy the command to the correct
backend server.


POP
---

POP is easy given that POP only allows access to the user’s INBOX.
When it comes to POP, the IMAP Aggregator acts just like a
multiplexor. The user authenticates to frontend server. The frontend
determines where the user’s INBOX is located and does a direct pass
through of the POP commands from the client to the appropriate backend
server.


MUPDATE
-------

The mupdate (slave) process (one per frontend) holds open a 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.


Analysis
========


Mailboxes Database
------------------

A benefit of having the mailbox information on the frontend is that
LIST is very cheap. The frontend servers can process this request
without having to contact each backend 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 to which backend it is being forwarded, so
lookups in the mailbox list are also quite frequent).


Failure Mode Analysis
---------------------


What happens when a backend 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 frontend server comes up?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   The only thing that needs to happen is for the frontend 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.


Summary of Benefits
-------------------

* **Availability** - By allowing multiple frontends, failures of the
  frontend only result in a reduction of capacity. Users currently
  connected still lose their session but can just reconnect to get
  back online.

     * The failure of the backends 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
       backend servers or the MUPDATE server to fail with no
       availability impact.

* **Load scalability** - No specific benchmarks have been run to
  show that this system actually performs better. However, it is clear
  that it scales to a larger number of users than a single server
  architecture would. Though, based on the fact that there are no
  further performance problems beyond when running a single machine,
  but handling about 20% more concurrent users, this is a success.

* **Management benefits** - As with AFS, administrators have the
  flexibility of placement of data on the servers, “live” move of data
  between servers,

* **User benefits** - The user only needs to know a single server
  name for configuration. The same name can be handed out to all
  users.

     * 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.


Futures
=======

It would be nice to be able to replicate the messages in a mailbox
among multiple servers and not just do partitioning for availability.

We are also evaluating using the aggregator to be able to provide
mailboxes to the user with a different backup policy or even different
“quality of service.” For example, we are looking to give users a
larger quota than default but not back up the servers where these
mailboxes exist.

There is possibility that LDAP could be used instead of MUPDATE.
However at this time the replication capabilities of LDAP are
insufficient for the needs of the Aggregator.

It would be nice if quotaroots had some better semantics with respect
to the murder (either make them first-class entities, or have them
apply across servers).


Appendices
==========


Appendix A: DNS Name Load Balancing
-----------------------------------

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.


Appendix B: IMAP Multiplexing
-----------------------------

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 backend server.

Multiplexors references:

   * Netscape Messaging Multiplexor

   * Paul Fleming’s IMAP Proxy

   * Perdition IMAP Proxy

   * Mirapoint Message Director - This is a hardware solution that
     also does content filtering.


Appendix C: Definitions
-----------------------

IMAP connection
   A single IMAP TCP/IP session with a single IMAP server is a
   “connection”.

client
   A client is a process on a remote computer that communicates with
   the set of servers distributing mail data, be they ACAP, IMAP,
   LDAP, or IMSP servers. A client opens one or more connections to
   various servers.

mailbox tree
   The collection of all mailboxes at a given site in a namespace is
   called the mailbox tree. Generally, the user Bovik’s personal data
   is found in "user.bovik".

mailboxes database
   A local database containing a list of mailboxes known to a
   particular server.

mailbox dataset
   The store of mailbox information on the ACAP server is the “mailbox
   dataset”.

mailbox operation
   The following IMAP commands are “mailbox operations”: CREATE,
   RENAME, DELETE, and SETACL.

MTA
   The mail transport agent (e.g. sendmail, postfix).

Murder of IMAP servers
   A grouping of IMAP servers. It sounded cool for crows so we decided
   to use it for IMAP servers as well.

quota operations
   The quota IMAP commands (GETQUOTA, GETQUOTAROOT, and SETQUOTA)
   operate on mailbox trees. In future versions of Cyrus, it is
   expected that a quotaroot will be a subset of a mailbox tree that
   resides on one partition on one server. For rationale, see section
   xxx.
