Author: Bron Gondwana

# Index API

## Intro

The Index API is implemented in `imap/index.h` and `imap/index.c`. It provides a snapshot view into the underlying mailbox (see the Mailbox API documentation which obeys IMAP semantics, as well as all the searching and sorting logic.

## Opening and closing

    struct index_state *state = NULL;
    struct index_init init;
    int r;
    const char *mboxname = "user.brong";

    memset(&init, 0, sizeof(struct index_init));
    init.userid = imapd_userid;
    init.authstate = imapd_authstate;
    init.out = imapd_out;

    r = index_open(mboxname, &init, &state);
    if (r) return r;

    do_stuff(state);

    index_close(&state);

The `index_init` interface sucks. So does passing lots of parameters. For now, this will do! Just pass NULL if you're only reading, or use the code already in imapd and you'll be fine.

## The Index Model

Ok - I think a few words about the index model and how it differs from direct mailbox access are needed! In the past, index.c used pointers directly into the mmaped `cyrus.index` file and maintained the old mmaped copy if an expunge took place. Under the namelock regime, this is no longer required because namelocks will avoid the file being re-written.

Also, memory is now cheap. Rather than using locks to ensure consistency, we just keep a copy of the `struct index_record` for even message in the index, stored in memory. Since these are about 100 bytes each, a 1 million email mailbox will take rougly 100Mb of memory. That's not too bad on a modern server, and that's a huge mailbox.

So - the model works like this:

- Create the index state or re-lock (`index_lock`) the mailbox on an existing index.
- call `index_refresh`
- if any changes are to be made (i.e. flag updates for a store, non-peek body fetch, expunge) then cycle through the refreshed state map and update the records which are affected.
- call `index_unlock` (unlock the underlying mailbox and commit the statuscache changes)

At this point the index lock is released and we have not yet generated any network traffic. Now start generating the response.

- if expunges are allowed, call `index_tellexpunge`
- call `index_tellchanges` to tell about all other changes
- return any response that the command itself required