My question concerns 0x7E8 response messages where the message is greater than 8 bytes.
I am sure OBD2 experts won't need much of the detail below but for the sake of clarity......
When a diagnostic device (eg scantool) sends a 0x7DF message with a Mode and PID value, most 0x7E8 responses consist of a single message of up to 8 bytes.
However some response messages require more than 8 bytes. eg request for the VIN and many vehicle manufacturer specific Mode/PID values. In this case ISO 15765-2 specifies that the first response 0x7E8 message has the code 0x10, Number of bytes, Mode, PID followed by data.
The diagnostic device then responds with a 0x7E0 flow control message according to the ISO 15765-2 spec. Noting that there is no information in that flow control message to identify which 0x7E8 response message is being flow controlled.
When the flow control message is received, the responding device then sends further 0x7E8 messages with the 0x2? code where "?" is the sequence number of the message. Again there is no information is those further 0x7E8 messages to identify which 0x7DF message Mode/PID is being responded to.
So my question is - Is it possible to have 2 or more diagnostic devices on an OBD2 bus sending 0x7DF messages where the response is more than 8 bytes?
As there is no information in either the 0x7E0 flow control message or the following 0x7E8 0x2? coded messages to identify the original 0x7DF request, multiple diagnostic devices on the OBD2 bus will not be able to identify which 0x7E8 response messages correspond to their request.
I have seen this 0x7E8 message confusion happen in practice with more than one scantool connected to a vehicle. But maybe I have missed something?
If you have more than one scan tool on the bus, then you need to take special care about orchestration, i.e. only the tester who initiated a question is allowed to send flow control ACKs.
A Cloud Function triggered by an HTTP request has a corresponding function-execution-id for each calling request (in the request and response header). It is used for tracing and viewing the log of a specific request in Stack Driver Logging. In my case, it is a string of 12 characters. When I continuously do HTTP requests to a cloud function and see the function-execution-id, I get the result below:
j8dorcyxyrwb
j8do4wolg4i3
j8do8bxu260m
j8do2xhqmr3s
j8dozkdlrjzp
j8doitxtpt29
j8dow25ri4on
On each line, the first 4 characters are the same "j8do" but the rest are different, so I wonder what is the structure of function-execution-id.
How was it generated?
The execution id is opaque, meaning that it doesn't contain any useful data. It is just a unique ID. How it was generated should not be of any issue to you, the consumer. From examination, it looks like it might be some time-based value similar to UUIDv1, but any code that you write that consumes these IDs should make no assumptions about how they were generated.
What are the good choice for API error code response pattern?
Instead of using different codes indicating different type of error
100001 // username not provided
100002 // password not provided
100003 // password too short
...
I see some other use patterns like the following (non-sequential) ...
20000
20001
20004
20015
Are there any other recommendations?
In my experience developing and using web services, I have found that a strategy of using a combination of top-level HTTP status codes and lower level API error codes work reasonably well. Note that the lower level API error codes don't need to be integers, but can be any enumeration. For a well-known public example, AWS Simple Email Service (SES) uses this strategy of using both HTTP status codes and API level error codes. You can see a sample error code response for SES here. Note that although SES uses XML response error payloads, this strategy works equally well for JSON response payloads.
In my experience, there are a few things that you need to keep in mind when using this strategy:
Strive to return the correct HTTP response code: HTTP is a ubiquitous protocol and is no doubt understood by your web container. Its response codes fit naturally into REST web services. As such, leverage it! If your web service encounters an error condition, you should do your best to return the correct HTTP status code in whose context, the API error code has meaning. One my biggest headaches in debugging issues with web services occur when developers just unconditionally throw arbitrary (usually runtime) exceptions back up the stack. The result is that everything gets returned back to the caller as an HTTP 500 (Internal Server Error) status code even when that's not the case (e.g. the client sends garbage data and the server just can't process it. Some common HTTP status codes you might want to design for include:
400 Bad Request: There is an issue with the client's request. Note this error isn't just used for things like broken JSON syntax in a POST request, but it is also a legitimate response code for semantic issues as well (i.e. the JSON request payload conformed to the prescribed schema, but there was an issue with the data in the payload, such as a number being negative when it is supposed to be only positive).
401 Unauthorized: The caller's credentials were invalid (i.e. authorization error).
403 Forbidden: The caller's credentials were valid, but their access level isn't sufficient to access the resource (i.e. authentication error).
404 Not Found: The resource of the URL doesn't exist.
500 Internal Server Error: Something bad happened inside the server itself, this error could be anything.
502 Bad Gateway: An error occurred when calling downstream service.
503 Service Unavailable: A useful response code for when you get hammered with a ton of "happy" customers who are inadvertently DDOS'ing your service.
504 Gateway Timeout: Like the 502 status code, but indicates a timeout instead of an actual error with the downstream service, per se.
HTTP response codes are the top-level codes, and API error codes only have meaning within that context: By this, I mean that your API error codes are only meaningful for certain HTTP response codes. For example, in the table of SES error codes, each error code is only tied to a single HTTP(S) response code. The error codes ConfigurationSetDoesNotExist and InvalidParameterValue only make sense when a 400 Bad Request is returned by SES - it wouldn't make sense to return these status codes when a 500 Internal Server Error is returned. Similarly, if you were writing a web service that called downstream services and databases, you might have a FooDownstreamServiceTimedOut error code that you would return with a 504 Gateway Timeout HTTP status code when a downstream web service call timed out to the "Foo" web service. You might also have a MyDatabaseError error code that you would return with a 500 Internal Server Error HTTP status code when your query to the internal DB fails.
Have a uniform error code schema irrespective of status codes: Your clients need to be able to process your error content programmatically. As such, it needs to conform to a certain schema. Ideally, your API error code schema should include the error code (i.e. name or ID, etc.). You also probably want to include a natural language description of the error code and the ID/GUID of the request that you are responding to. For an example of an error schema, see this sample AWS SES response and schema. Additionally, you might also want to consider returning a client ID in the response. This is as much for your own benefit as the client's since it can help you drill down into the data to see if one particular client is getting a glut of particular errors vs. your other clients.
Consider returning natural language descriptions of the error codes in the response: To make things easier on your clients, you might want to consider not just returning the error code in the error payload, but a natural language description as well. This kind of behavior can immediately help confused and busy engineers who really don't care that much about your service quickly diagnose what's happening so that they can resolve the issue ASAP. btw, enabling engineers to quickly diagnose issues with your service increases the all-important "uptime" metric that your customers and managers will no doubt care about.
Don't feel obliged to use integers, use enumerations instead: The notion of "error codes" conjures up images of outdated technologies and codebooks where you had to look up what an error meant. It arose from the programming dark ages when engineers needed to fit all possible errors into a byte of space, or a nibble or whatever. Those days are gone, and your error code can be a string, likely without any meaningful impact on performance. You might as well take advantage and make the error code meaningful, as a means of keeping things simple.
Return info to clients that they might need to debug, but be mindful of security: If possible, return whatever debug info your clients may need. However, if your service potentially deals with sensitive information such as credit card numbers and the like, you probably don't want to pass that info around for obvious reasons.
Hope that helps.
A recommendation by the IETF (internet standards body) is using the application/problem+json mediatype.
Notable is that they don't use random numbers, they use strings (specifically uris) to identify errors.
This is a subjective question, but even if you don't use their format, I'd argue that username-not-provided is better in almost every way to 100001.
I would say this heavily depends on what kind of API you're providing.
I were to always include a field called ack or something similar in every response that has three states: failure, warning, success. Success obviously being everything went well. On warning, the request went through and the JSON will contain the expected output, but it will also include a warning string, or even better in case multiple warnings could occur an array called errors which consists of multiple objects containg code, string and type. This array will also be returned in case of failure, and nothing else but this array.
The array contains one object per error or warning, having a code (I would suggest going with your initial idea of 10001, 10002, ...) and a string explaining the error in a very short phrase (e.g. Username contains invalid characters). The type is either error or warning, which is useful in case of a failure ack that contains not only errors but also warnings.
This makes it easy to look up errors by their code (I would provide a page, also with an API, that contains all the error codes in a table along with their short and long description plus common causes/fixes/etc. - All this information should also be available via an API where they can be accessed by providing the error code) while still having a quick short text response so the user can tell what's wrong in most cases without having to look up the error.
This also allows for easy output of warnings and errors to the end user, not just the developers. Using my idea with the API call to get informations about an error, developers using your API could easily provide full information about errors to end-users when needed (including causes/fixes/whatever you see fit).
Instead of writing your own API standard from scratch adopt one of the already available, for example the JSON API standard:
If you’ve ever argued with your team about the way your JSON responses should be formatted, JSON API can be your anti-bikeshedding tool.
By following shared conventions, you can increase productivity, take advantage of generalized tooling, and focus on what matters: your application.
Clients built around JSON API are able to take advantage of its features around efficiently caching responses, sometimes eliminating network requests entirely.
If you decide to go with JSON API it has a section dedicated to errors and a few error examples.
For many years, many developent companies have created things like bitmask for errors, so they can encode multiple variables inside the error:
000 - all ok
001 - something failed with X
010 - something failed with Y
011 - something failed with X and Y
100 - something failed with Z
101 - something failed with X and Z
The limitation is that that limits the error space into however many bytes you decide on the encoding, like 16 or 32 possible combinations, it may be enough for you, or not.
You see this being common in COM+
https://learn.microsoft.com/en-us/windows/desktop/com/com-error-codes-1
I hope this helps.
How can I, in my application, sent a message as input to my application using message queue?
It is showing as number of messages in message queue is zero.
Providing that you use Linux and C language, a simple example how POSIX message queue can be used to pass data between separate threads or processes can be found here.
We are using NService bus for our messaging framework.Sometime the message is not coming as par the sequence of sending .Sometimes last message is coming first and than later first message.
Please help me out Thanks
The nature of NServiceBus does not guarantee that messages will be received in the order they were sent. Each message is meant to be processed independently.
If an action can only be undertaken after two related messages arrive, then you need to utilize a Saga
Edit in response to first comment:
You mention you're sending the same message in chunks. Does this mean that you have a large payload that you have to split up into multiple parts to transmit via MSMQ?
If so, you have a few options:
Store the payload out of band, in a database or file system, and only put enough data in one message (an ID or file system path) to load the data from the message handler.
Make the message a MessagePart that contains a BundleID, PartNumber, TotalParts, and PayloadChunk. Then, create a saga for MessagePart that stores each part and when all parts have been received, reconstitute the chunks together and do what you need. Of course, if you need to then send the resulting large object back onto the Bus, this would get annoying really quickly, so then the out-of-band option would look much more attractive.
In any case, there are a ton of reasons why any MSMQ message, not just NServiceBus messages, could arrive out of order, so you have to be able to deal with it.
Would Bus.Sending a collection of Imessages work? NServiceBus allows batching of messages