Giampaolo,
> I guess it's better you don't use the mail_id field as a primary key: it
> may not be unique in your db. Think, in example, to messages sent to
> destinations defined as a list or an alias in your system.
Don't confuse mail_id with a Message-ID header.
mail_id is guaranteed by amavisd to be unique among all records
currently present in a database.
amavisd-new-2.3.0 release notes:
- introduce a concept of 'mail_id', which is similar to the am_id as reported
in the log and elsewhere (e.g. 92598-11-5), but has much stronger long-term
uniqueness property and can be used for the purpose of uniquely identifying
a quarantined mail, or for other uses. The mail_id is a 12-character string
consisting of characters [A-Za-z0-9+-] (like base64, except for a '/' being
substituted by a '+'), guaranteed to start and to end with an alphanumeric
character (i.e. not with '+' or '-'). It is derived by cryptographically
strong method (MD5), cumulatively collecting entropy during the life of
child processes, folding-in entropy from processed mail and other cheaply
accessible sources, collected when an opportunity arises (e.g. file system
file-IDs, SA results etc), without placing a burden on system sources of
randomness (see RFC 4086). Note that MD5 has been demonstrated to have some
weaknesses, but we are not talking about cryptographic attacks here, but
rather about spreading messages which have no inherent intention of causing
collisions.
The mail_id carries 71.9 bits of information (subject to the quality of
sources). For a high-end system that keeps a year's worth of mail messages
in evidence (e.g. in quarantine) and receives 10e6 messages each day
(20..200 TB of yearly mail contents), the probability of a mail_id
collision happening during one year (while gradually displacing an entire
collection with a new set of IDs) is n^2/m = 0.3 %
(10e6 * 365)^2 / (62 * (64^(12-2)) * 62) = (10e6 * 365)^2 / 2^71.9 = 0.003
Eventual clash is still possible and needs to be detected, but a re-tried
mail delivery attempt is acceptable if its probability is low, as each mail
processing rolls a dice again. On a smallish system receiving 10.000 mail
messages daily an 8-character mail_id would suffice, but the savings are
not worth the trouble of providing configuration flexibility.
Paired with a mail_id there is its companion secret_id generated for each
message, such that mail_id can be derived from secret_id and pairing checked,
but not the other way around. The purpose of secret_id is not yet fully
developed, but can serve as a 'ticket', granting user a right to release
a quarantined message addressed to him.
Mark
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