HostDB is a cache of DNS results. It is used to increase performance by aggregating address resolution across transactions. HostDB also stores state information for specific IP addresses.
The primary operation for HostDB is to resolve a fully qualified domain name (“FQDN”). As noted each FQDN is associated with a single record. Each record has an array of items. When a resolution request is made the database is checked to see if the record is already present. If so, it is served. Otherwise a DNS request is made. When the nameserver replies a record is created, added to the database, and then returned to the requestor.
Each info tracks several status values for its corresponding upstream. These are
Last failure time
The HTTP version is tracked from responses and provides a mechanism to make intelligent guesses about the protocol to use to the upstream.
The last failure time tracks when the last connection failure to the info occurred and doubles as
a flag, where a value of
TS_TIME_ZERO indicates a live target and any other value indicates a
If an info is marked dead (has a non-zero last failure time) there is a “fail window” during which no connections are permitted. After this time the info is considered to be a “zombie”. If all infos for a record are dead then a specific error message is generated (body factory tag “connect#all_dead”). Otherwise if the selected info is a zombie, a request is permitted but the zombie is immediately marked dead again, preventing any additional requests until either the fail window has passed or the single connection succeeds. A successful connection clears the last file time and the info becomes alive.
DNS results are stored in a global hash table as instances of
HostDBRecord. Each record stores
the results of a single query. These records are not updated with new DNS results - instead a new
record instance is created and replaces the previous instance in the table. The records are
reference counted so such a replacement doesn’t invalidate the old record if the latter is still
being accessed. Some specific dynamic data is migrated from the old record to the new one, such as
the failure status of the upstreams in the record.
In each record is a variable length array of items, instances of
HostDBInfo, one for each
IP address in the record. This is called the “round robin” data for historical reasons. For SRV
records there is an additional storage area in the record that is used to store the SRV names.
The round robin data is accessed by using an offset and count in the base record. For SRV records
each record has an offset, relative to that
HostDBInfo instance, for its own name in the name
State information for the outbound connection has been moved to a refurbished
ResolveInfo. As much as possible relevant state information has been moved from the
HttpSM to this structure. This is intended for future work where the state machine deals only
with upstream transactions and not sessions.
ResolveInfo may contain a reference to a HostDB record, which preserves the record even if it is
replaced due to DNS queries in other transactions. The record is not required as the resolution
information can be supplied directly without DNS or HostDB, e.g. a plugin sets the upstream address
resolved_p flag indicates if the current information is valid and ready to be
used or not. A result of this is there is no longer a specific holder for API provided addresses -
the interface now puts the address in the
ResolveInfo and marks it as resolved. This prevents
further DNS / HostDB lookups and the address is used as is.
The upstream port is a bit tricky and should be cleaned up. Currently value in
determines the port if set. If not, then the port in
addr is used.
Metadata about the source of the resolved address.’
Use default resolution. This is the initial state.
Use HostDB to resolve the target key.
Use the client supplied target address. This is used for transparent connections - the upstream address is obtained from the inbound connection. May fail over to HostDB.
Use HostDB to resolve the target key.
Use the client supplied target address.
Use the address provided via the plugin API.
The parallel values for using HostDB and the client target address are to control fail over on connection failure. The
TRY_values can fail over to another style, but the
USE_values cannot. This prevents cycles of style changes by having any
TRY_value fail over to a
USE_value, at which point it can no longer change. Note there is no
TRY_API- if a plugin sets the upstream address that is locked in.
Currently if an upstream is marked down connections are still permitted, the only change is the number of retries. This has caused operational problems where dead systems are flooded with requests which, despite the timeouts, accumulate in ATS until ATS runs out of memory (there were instances of over 800K pending transactions). This also made it hard to bring the upstreams back online. With these changes requests to dead upstreams are strongly rate limited and other transactions are immediately terminated with a 502 response, protecting both the upstream and ATS.
There is still some work to be done in future PRs.
The fail window and the zombie window should be separate values. It is quite reasonable to want to configure a very short fail window (possibly 0) with a moderately long zombie window so that probing connections can immediately start going upstream at a low rate.
Failing an upstream should be more loosely connected to transactions. Currently there is a one to one relationship where failure is defined as the failure of a specific transaction to connect. There are situations where the number of connections attempts for mark a failure is should be larger than the number of retries for a single transaction. For transiently busy upstreams and low latency requests it can be reasonable to tune the per transaction timeout low with no retries but this then risks marking down upstreams that were merely a bit slow at a given moment.
Parallel DNS requests should be supported. This is for both cross family requests and for split DNS.
It would be nice to be able to do the probing connections to an upstream using synthetic requests instead of burning actual user requests. What would be needed is a handoff from ATS to the probe to indicate a particular upstream is considered down, at which point active health checks are done until the upstream is once again alive, at which point this is handed off back to ATS.
This version has several major architectural changes from the previous version.
The data is split into records and info, not handled as a variant of a single data type. This provides a noticeable simplification of the code.
Single and multiple address results are treated identically - a singleton is simply a multiple of size 1. This yeilds a major simplification of the implementation.
Connections are throttled to dead upstreams, allowing only a single connection attempt per fail window timing until a connection succeeds.
Timing information is stored in
std::chronodata types instead of proprietary types.
State information has been promoted to atomics and updates are immediate rather than scheduled. This also means the data in the state machine is a reference to a shared object, not a local copy. The promotion was necessary to coordinate zombie connections to dead upstreams across transactions.
The “resolve key” is now a separate data object from the HTTP request. This is a subtle but major change. The effect is requests can be routed to different upstreams without changing the request. Parent selection can be greatly simplified as it become merely a matter of setting the resolve key, rather than having a completely different code path.