6.3 KiB
struct lws_sequencer introduction
Often a single network action like a client GET is just part of a larger series of actions, perhaps involving different connections.
Since lws operates inside an event loop, if the outer sequencing doesn't, it can be awkward to synchronize these steps with what's happening on the network with a particular connection on the event loop thread.
struct lws_sequencer provides a generic way to stage multi-step
operations from inside the event loop. Because it participates
in the event loop similar to a wsi, it always operates from the
service thread context and can access structures that share the
service thread without locking. It can also provide its own
higher-level timeout handling.
Naturally you can have many of them running in the same event loop operating independently.
Sequencers themselves bind to a pt (per-thread) service thread,
by default there's only one of these and it's the same as saying
they bind to an lws_context. The sequencer callback may create
wsi which in turn are bound to a vhost, but the sequencer itself
is above all that.
Sequencer timeouts
The sequencer additionally maintains its own second-resolution timeout checked by lws for the step being sequenced... this is independent of any lws wsi timeouts which tend to be set and reset for very short-term timeout protection inside one transaction.
The sequencer timeout operates separately and above any wsi timeout, and is typically only reset by the sequencer callback when it receives an event indicating a step completed or failed, or it sets up the next sequence step.
If the sequencer timeout expires, then the sequencer receives a queued
LWSSEQ_TIMED_OUT message informing it, and it can take corrective action
or schedule a retry of the step. This message is queued and sent normally
under the service thread context and in order of receipt.
Unlike lws timeouts which force the wsi to close, the sequencer timeout
only sends the message. This allows the timeout to be used to, eg, wait
out a retry cooloff period and then start the retry when the
LWSSEQ_TIMED_OUT is received, according to the state of the sequencer.
Creating an struct lws_sequencer
typedef struct lws_seq_info {
struct lws_context *context; /* lws_context for seq */
int tsi; /* thread service idx */
size_t user_size; /* size of user alloc */
void **puser; /* place ptr to user */
lws_seq_event_cb cb; /* seq callback */
const char *name; /* seq name */
const lws_retry_bo_t *retry; /* retry policy */
} lws_seq_info_t;
struct lws_sequencer *
lws_sequencer_create(lws_seq_info_t *info);
When created, in lws the sequencer objects are bound to a 'per-thread',
which is by default the same as to say bound to the lws_context. You
can tag them with an opaque user data pointer, and they are also bound to
a user-specified callback which handles sequencer events
typedef int (*lws_seq_event_cb)(struct lws_sequencer *seq, void *user_data,
lws_seq_events_t event, void *data);
struct lws_sequencer objects are private to lws and opaque to the user. A small
set of apis lets you perform operations on the pointer returned by the
create api.
Queueing events on a sequencer
Each sequencer object can be passed "events", which are held on a per-sequencer
queue and handled strictly in the order they arrived on subsequent event loops.
LWSSEQ_CREATED and LWSSEQ_DESTROYED events are produced by lws reflecting
the sequencer's lifecycle, but otherwise the event indexes have a user-defined
meaning and are queued on the sequencer by user code for eventual consumption
by user code in the sequencer callback.
Pending events are removed from the sequencer queues and sent to the sequencer callback from inside the event loop at a rate of one per event loop wait.
Destroying sequencers
struct lws_sequencer objects are cleaned up during context destruction if they are
still around.
Normally the sequencer callback receives a queued message that
informs it that it's either failed at the current step, or succeeded and that
was the last step, and requests that it should be destroyed by returning
LWSSEQ_RET_DESTROY from the sequencer callback.
Lifecycle considerations
Sequencers may spawn additional assets like client wsi as part of the sequenced actions... the lifecycle of the sequencer and the assets overlap but do not necessarily depend on each other... that is a wsi created by the sequencer may outlive the sequencer.
It's important therefore to detach assets from the sequencer and the sequencer
from the assets when each step is over and the asset is "out of scope" for the
sequencer. It doesn't necessarily mean closing the assets, just making sure
pointers are invalidated. For example, if a client wsi held a pointer to the
sequencer as its .user_data, when the wsi is out of scope for the sequencer
it can set it to NULL, eg, lws_set_wsi_user(wsi, NULL);.
Under some conditions wsi may want to hang around a bit to see if there is a subsequent client wsi transaction they can be reused on. They will clean themselves up when they time out.
Watching wsi lifecycle from a sequencer
When a sequencer is creating a wsi as part of its sequence, it will be very interested in lifecycle events. At client wsi creation time, the sequencer callback can set info->seq to itself in order to receive lifecycle messages about its wsi.
| message | meaning |
|---|---|
LWSSEQ_WSI_CONNECTED |
The wsi has become connected |
LWSSEQ_WSI_CONN_FAIL |
The wsi has failed to connect |
LWSSEQ_WSI_CONN_CLOSE |
The wsi had been connected, but has now closed |
By receiving these, the sequencer can understand when it should attempt reconnections or that it cannot progress the sequence.
When dealing with wsi that were created by the sequencer, they may close at
any time, eg, be closed by the remote peer or an intermediary. The
LWSSEQ_WSI_CONN_CLOSE message may have been queued but since they are
strictly handled in the order they arrived, before it was
handled an earlier message may want to cause some api to be called on
the now-free()-d wsi. To detect this situation safely, there is a
sequencer api lws_sequencer_check_wsi() which peeks the message
buffer and returns nonzero if it later contains an LWSSEQ_WSI_CONN_CLOSE
already.