Detect when deterministically generated container codes collide, and handle appropriately

[pmalmsten]

To minimize the total size of generated HTTP headers, the combination of account URI, database name, and container name are hashed, and the resulting hash is truncated to a shorter integer (currently between 0 and 1,000,000).

The probability of collisions is relatively low, but the possibility of it happening is may be non-trivial when large numbers of containers are involved (e.g. 50 containers).

We should do the right thing if/when that happens. To start brainstorming, this might include some combination of:

• If running in a development environment, throw an exception - otherwise, log an error and drop the conflicting session information
• Provide a way for users to override the automatic code assignment behavior for specific account/database/container combinations, in order to work around conflicts when they are discovered
• Or, as an alternative to the other workarounds, increase the set of possible container codes until collisions are sufficiently unlikely to be ignored entirely. (Not a particularly attractive choice, since this would mean silent problems for users, and would increase the size of HTTP headers for everyone using cookies).

[pmalmsten]

For context, the probability that a collision would occur among 50 containers using a hash code range of 0 - 1,000,000 (assuming that the hash function in use is perfectly uniform) is:

(1 - (1/1000000)^50×(1000000!)/((1000000 - 50)!)) = 0.00122, or 0.122%

That probability is already relatively small (given that using >= 50 different Cosmos DB containers in a single app is probably uncommon).