Imagine you have a replay observable, created through one of the Replay overloads.
Is there a variant of Replay that produces a replay observable with a method that lets me remove an item/message/element from the replay buffer?
I need the replay observable in order to avoid a race condition: two observables A and B produce related messages. If A produces a message m, then B may follow suit with a corresponding message m' (but that's optional).
In order to filter out m' from all of B's messages, the subscriber to A registers a subscriber with B (filtered for the exact m' message), since only when m is received will the program be able to predict what a potentially incoming m' will look like.
However, sometimes m' arrives even before the subscriber to A was scheduled to run, meaning there is no subscriber for m' registered yet. I can solve this by wrapping B as a replay observable, but I want to be able to clean up processed messages from the replay buffer, because I know each of B's messages will be processed by exactly one subscriber.
var result = foo.Replay().Where(x => x != skip);
Related
I am trying to understand the exact semantics of erlang:trace_delivered/1 in order to determine whether this function can be suitably used to solve a problem I am currently facing. The problem is as follows.
Suppose there is a tracee process X, a tracer Y tracing X, and a third process, Z. Tracer Y is initially tracing X. Process Z is tasked with the responsibility of stopping Y from tracing X by calling erlang:trace(X, false). This call is arbitrarily effected by Z whilst X is running. Thereafter, Z is also to issue a special message stopped to tracer Y which signals to Y that Z has indeed stopped Y from tracing X.
I would like to guarantee that the special stopped message is delivered to Y's mailbox after all other trace messages have been delivered to Y. To my knowledge, Erlang does not guarantee the ordering of messages sent by different processes to a single process. Concretely in my case this means that tracer Y can, for instance, receive the stopped issued by Z before the trace event messages due to tracee X. I read about erlang:trace_delivered/1, and was planning to address my problem using the following code inside the implementation of Z:
...
erlang:trace(X, false),
Ref = erlang:trace_delivered(X),
receive
{trace_delivered, X, Ref} ->
Y ! stopped
end.
...
A somewhat similar example is provided in the docs (quoted below):
Example: Process A is Tracee, port B is tracer, and process C is the port owner of B. C wants to close B when A exits. To ensure that the trace is not truncated, C can call erlang:trace_delivered(A) when A exits, and wait for message {trace_delivered, A, Ref} before closing B.
My example differs from the one in the docs in these two respects:
Process C knows the exact point of execution of A prior to invoking erlang:trace_delivered(A); in my case, I do not know the point X is in when erlang:trace_delivered(X) is invoked by Z.
By contrast to process C, my process Z switches off tracing on X before invoking erlang:trace_delivered(X).
What are the semantics of erlang:trace_delivered/1 in this case?
Does it still guarantee that {trace_delivered, X, Ref} is received by process Z after all trace messages have been delivered to tracer Y?
And does erlang:trace_delivered/1 automatically keep track of the point at which the execution of tracee X is at to be able to provide the aforementioned guarantee?
Your help is greatly appreciated!
You are correct about the ordering of messages. If process A sends multiple messages to process B they will be guaranteed to arrive in order. If A sends multiple messages to processes B and C the message order will only be guaranteed per-process. So for example:
A sends B message 1
A sends C message 2
A sends B message 3
A sends C message 4
The only guarantees in this scenario is that B will receive message 1 before message 3, and C will receive message 2 before message 4.
To answer your questions:
No, if erlang:trace_delivered/1 is invoked prior to other trace events being generated those messages will arrive later. The docs only guarantee that prior trace messages are delivered prior to {trace_delivered, ...}:
When it is guaranteed that all trace messages are delivered to the tracer up to the point that Tracee reached at the time of the call to erlang:trace_delivered(Tracee), then a {trace_delivered, Tracee, Ref} message is sent to the caller of erlang:trace_delivered(Tracee) .
The guarantees around erlang:trace_delivered/1 always hold true. But it doesn't guarantee that {trace_delivered, ...} will always be the last message.
If one server receives multiple requests from one process by using pid ! Msg, but the process time for each request is different, then how to guarantee the sender receives the reply in order?
From the Erlang FAQ:
10.8 Is the order of message reception guaranteed?
Yes, but only within one process.
If there is a live process and you send it message A and then message B, it's guaranteed that if message B arrived, message A arrived before it.
On the other hand, imagine processes P, Q and R. P sends message A to Q, and then message B to R. There is no guarantee that A arrives before B. (Distributed Erlang would have a pretty tough time if this was required!)
That is, if the server processes the requests in the order they arrive, and sends the responses in the order the requests were processed, then the sender will receive the responses in order.
the Erlang receive clause can do pattern matching. So what you can do is create a reference for each message that you want to receive and then pattern match on that reference.
Check out this gist if you look at line 26 you will see that the receive clause is waiting for a message with a specific pid. In this case the messages will arrive in an arbitrary order but by virtue of this receive, they will be put into order.
I would like to design a process hierarchy where there is a a parent process P which acts like a gatekeeper and delegates the work(messages/events from its client processes) to it's children processes C1,C2..Cn which collaborate with each other and may send the result back to P. The children processes cannot talk to any process outside, only P.
The challenge is that though P may have multiple messages from its clients, it should accept only one message, delegate the work to C1..Cn and ONLY accept the next message from its clients
when all its children processes are done(or idle) and there are no more messages circulating between C1 to Cn.
P finishes accepting messages from C1..Cn so that it can return the result to its clients
Constraints:
Idle for me is when they are waiting with a receive (blocking) or simply exited.
C1 to Cn are finite state machines. Some or all of them may send messages back to C. Or there may be no messages to be sent back to C. Even if no messages are sent back to C, C has to figure out that all of them are done with no messages between them.
If any of C1 to Cn have been pre-empted, then it is considered busy(this may be obvious but I thought I'll put it here for completion) and C will not receive the next message
Is there an OTP pattern or library which will do this for me (before I hack something?). I know that process_info can let me know if the mailbox of a process are empty and I could keep on checking the children's mailboxes from P but it would be unnecessary polling from P.
EDIT GENERAL: I am trying to implement a reactive variant of Flow Based Programming on the Erlang platform. This has the notion of 'hierarchical processes' or composites which themselves may contain composite processes until we reach some boxes of actual code...I am going to research(looking at monitor,process_info,process_flag) but I wanted to respond to your excellent answers
EDIT RECURSIVE PARENTS: Each of C1 and Cn can themselves be parent/composite processes. If I just spawn processes and let them exit immediately, I'll have to create the chain of Composites everytime as C1..Cn may themselves be composites (which spawn composites..and so on). Finally, when we reach a leaf box(which is not a composite node), they are supposed to be finite state machines.. so I'm not sure of spawning and making them exit quickly if the are FSMs.
EDIT TKOWAL: Since I am trying to create a generic parent/composite process, it does not know 'when' the task ends. All it does is relay the messages it receives from its children to it's siblings with the 'constraint' that it will not accept the next message from its client/siblings until its children are 'done'. The children C1..Cn may send not just one but many messages. I understand from your proposal, that wait_for_task_finish will stop blocking the moment it gets the first message. But more messages may be emitted too by P's children. P should wait for all messages. Also, having a task_end symbol will not work for the same reason(i.e. multiple messages possible from the children)
Given how inexpensive it is to start up Erlang processes, your gatekeeper could start new children for each incoming task, and then wait for them all to exit normally once they complete their work.
But in general, it sounds like you're looking for a process pool. There are a few of these already available, such as poolboy and sidejob. Pools can be harder to get right than you think, so I advise using an existing proven pool implementation before attempting to write your own.
After edits, this became entirely different question, so I am posting new answer.
If you are trying to write Flow Based Programming, then you are probably solving wrong problem. FBP is effective, because almost everything is asynchronous and you start processing next request immediately after you finished with previous one.
So, the answer is - don't wait for children to finish:
In FBP, there is no time dependencies between the components. So if I
have a chunk of data, it should be able to flow from one end of the
diagram to the other regardless of how any other pieces of data are
being handled. In order to program an FBP system, you have to minimize
your dependencies.
source
When creating parent and children, you know all the connections between blocks, so just configure children to send processed data directly to next block. For example: P1 has children C1 and C2. You send message to P1, it delegates it to C1, packet flows couple of times between C1 and C2 and after that, C1 or C2 sends it directly to P2.
Blocks should be stateless. They output should not depend on previous requests, so even if C1 and C2 are processing data from two different requests to P1 - it is OK. There could be situations, where P1 gets data packet D1 and then D2, but will output answers in different order R2 and then R1. It is also OK. You can use Erlang reference to tag messages and then check, which response is from which request.
I don't think, there is ready library for that, but it is really easy to hack, unless I missed something. Your P process should look like this:
ready_for_next_task() ->
receive
{task, Task, CallerPid} ->
send_task_to_workers(Task)
end,
wait_for_task_finish(CallerPid).
wait_for_task_finish(CallerPid) ->
receive
{task_end, Response} ->
CallerPid ! Response
end,
ready_for_next_task().
In wait_for_task_finish/1 you have only one clause for receive, so it will not accept next task, until current one is finished. If you are waiting for multiple responses from workers, you can simply add second clause to receive with some partial response and call wait_for_task_finish/1 recursively.
It is always better to have some indicator, that the processing ended, because you don't have guarantees on message delivery time. This means, that you could check, that all processes currently are waiting for message and think, that they ended processing, but actually, they did not started yet or one of them send message to other and you caught them before the second one had it in message box.
If the processes C1..Cn have only parts of actual work and don't know about the progress, than the gatekeeper P should know how many parts there were, receive all of them one by one and then call ready_for_next_task/1.
While reading the ERTS user's guide, I found this section:
The only signal ordering guarantee given is the following. If an entity sends multiple signals to the same destination
entity, the order will be preserved. That is, if A sends a signal S1 to B, and later sends the signal S2 to B, S1 is
guaranteed not to arrive after S2.
I've also happened across this while doing further research googling:
Erlang Reference Manual, 13.5:
Message sending is asynchronous and safe, the message is guaranteed to eventually reach the recipient, provided that the recipient exists.
That seems very vague and I'd like to know what guarantees I can rely on in the following scenario:
A,B are processes on two different nodes.
Assume A does not crash and B was a valid node at some point.
A and B monitor each other.
A sends messages M1,M2,M3 to B
In the above scenario, is it possible that B receives M1,M3 (M2 is dropped),
without any sort of 'DOWN'/'EXIT'/heartbeat timeout being received at A?
There are no other guarantees other than the ordering guarantee. Note that by default you don't even know who the sender is, unless the sender encodes this in the message.
Your example could happen:
A sends M1 and M2
B receives M1
The node on which B resides gets disconnected
The node on which B resides comes up again
A sends M3 to B
B receives M3
M2 can be lost on the network link in this scenario. It is highly unlikely this happens, but it can happen. The usual trick is to have some kind of notion of such errors. Either by having a timeout trigger, or by monitoring the node or Pid which is the recipient of the message.
Updated scenario:
In the updated scenario, provided I read it correctly, then A would get a 'DOWN' style message at some point, and likewise, it would get a message telling you that the node is up again, if you monitor the node.
Though often, such things are better modeled using an idempotent protocol if at all possible.
Reading through the erlang mailing-list and the academic faq, it seems like there are a few guarantees provided by the ERTS implementation, however I was not able to determine whether or not they are guaranteed at a language/specification level, too.
If you assume TCP is "reliable", then the current implementation guarantees that
given A,B are processes on different nodes (&hosts) and A monitors B, A sends to B, assuming A doesn't crash, any message delivery failures* between the two nodes or host/node/process failures on B will lead to A getting a 'DOWN' message (or 'EXIT' in the case of links). [ See 1 and 2 ]
*From what I have read on the mailing-list thread , this property is almost entirely based on the fact that TCP is used, so "message delivery failure" means any situation where TCP decides that a failure has occurred/the connection needs to be closed.
The academic faq talks about this like it's also a language/specification level guarantee, however I couldn't yet find anything to back that up.
Are messages processed in a first-come-first-serve basis or are they sorted by timestamp or something like that?
Order of messages is preserved between a process and another one. Reading from the FAQ:
10.9 Is the order of message reception guaranteed?
Yes, but only within one process.
If there is a live process and you send it message A and then message
B, it's guaranteed that if message B arrived, message A arrived before
it.
On the other hand, imagine processes P, Q and R. P sends message A to
Q, and then message B to R. There is no guarantee that A arrives
before B. (Distributed Erlang would have a pretty tough time if this
was required!)
#knutin is right regarding how you can consume messages within a process. As an addition, note that you might use two subsequent receive statements to ensure that a certain message is consumed after another one:
receive
first ->
do_first()
end,
receive
second ->
do_second()
end
The receive statement is blocking. This will ensure that you never do_second() before you do_first(). The difference from #knutin's second solution is that, in that case, if something not important arrives just before an important one, you queue the important bit.
The mailbox is always kept in the order the messages arrived.
However, the order the messages are consumed is determined by your code.
If you have a plain process with a generic receive clause that receives anything, the order you get messages are the same as the order they arrived in.
loop() ->
receive
Any ->
do_something(Any),
loop()
end.
However, if you have a selective receive with match clauses, it will search the mailbox for messages of this specific type and consume the first matching message, effectively skipping non-matching messages. In the following example, if there are messages tagged as important in the queue, they will be processed before any other message. Note: Matching like this will search all messages in the queue, which is a problem for many messages. There has been some developments in this area, but I'm not up to speed.
loop() ->
receive
{important, Stuff} ->
do_something_important(Stuff),
loop();
Any ->
do_something(Any)
loop()
end.
to further define the answer, I would like to point the fact that, as stated above, messages that don't pattern match are skipped, but in reality they are simply put aside and then reintroduced in order (so first that any other message arrived after the not matching messages) for next receive pattern matching.
This problem really shows its worst when you have, for example, a gen_server behaviour module because in this case, having always the same pattern matching call scheme, messages not in scope are going to flood message queue unless you define a (ugly and error prone, IMHO) match-all pattern like:
receive
... -> ...;
... -> ...;
MatchAllPatterns -> ok.
end