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# gRPC internal workings

- Headers
- Trailers
- Accessing the gRPC context
- Accessing the gRPC HTTP context
- Status codes in gRPC

# Headers

HTTP protocol level headers will work the same

# Accessing the HTTP context

We can access the HTTP context by obtaining it from `ServerCallContext context`, by calling `context.GetHttpContext()`. The properties exposed in the returned object are the ones we know from Web API or MVC.

# Trailers

Trailers are sent to the client after RPC finishes.

Adding trailers in the response

      Metadata.Entry myHeader = new Metadata.Entry("my-fake-header", "grpc-header");
      context.ResponseTrailers.Add(myHeader);

Obtaining trailers from the response

    var trailers = response.GetTrailers();
    var myValue = trailers.GetValue("my-fake-header");

You can obtain them in server streaming reponses.

# Accessing the gRPC context

We can access the gRPC call context using the mandatory parameter in our service methods: `ServerCallContext context` This is related only to the gRPC call

    public Metadata RequestHeaders => RequestHeadersCore;

        /// <summary>Cancellation token signals when call is cancelled. It is also triggered when the deadline is exceeeded or there was some other error (e.g. network problem).</summary>
        public CancellationToken CancellationToken => CancellationTokenCore;

        /// <summary>Trailers to send back to client after RPC finishes.</summary>
        public Metadata ResponseTrailers => ResponseTrailersCore;

        /// <summary> Status to send back to client after RPC finishes.</summary>
        public Status Status

# Status codes in gRPC

We are familiar with HTTP status codes, like 200 OK, 400, 404, etc. In gRPC these are doubled(or some of them are replaced by)

Status codes enum

    public enum StatusCode
        {
            OK = 0,
            Cancelled = 1,
            Unknown = 2,
            InvalidArgument = 3,
            DeadlineExceeded = 4,
            NotFound = 5,
            AlreadyExists = 6,
            PermissionDenied = 7,
            ResourceExhausted = 8,
            FailedPrecondition = 9,
            Aborted = 10,
            OutOfRange = 11,
            Unimplemented = 12,
            Internal = 13,
            Unavailable = 14,
            DataLoss = 15,
            UNAUTHENTICATED= 16
        }

Code | Number | Description
--- | --- | ---
OK | 0 | Not an error; returned on success.
CANCELLED | 1 | The operation was cancelled, typically by the caller.
UNKNOWN | 2 | Unknown error. For example, this error may be returned when a `Status` value received from another address space belongs to an error space that is not known in this address space. Also errors raised by APIs that do not return enough error information may be converted to this error.
INVALID_ARGUMENT | 3 | The client specified an invalid argument. Note that this differs from `FAILED_PRECONDITION`. `INVALID_ARGUMENT` indicates arguments that are problematic regardless of the state of the system (e.g., a malformed file name).
DEADLINE_EXCEEDED | 4 | The deadline expired before the operation could complete. For operations that change the state of the system, this error may be returned even if the operation has completed successfully. For example, a successful response from a server could have been delayed long
NOT_FOUND | 5 | Some requested entity (e.g., file or directory) was not found. Note to server developers: if a request is denied for an entire class of users, such as gradual feature rollout or undocumented allowlist, `NOT_FOUND` may be used. If a request is denied for some users within a class of users, such as user-based access control, `PERMISSION_DENIED` must be used.
ALREADY_EXISTS | 6 | The entity that a client attempted to create (e.g., file or directory) already exists.
PERMISSION_DENIED | 7 | The caller does not have permission to execute the specified operation. `PERMISSION_DENIED` must not be used for rejections caused by exhausting some resource (use `RESOURCE_EXHAUSTED` instead for those errors). `PERMISSION_DENIED` must not be used if the caller can not be identified (use `UNAUTHENTICATED` instead for those errors). This error code does not imply the request is valid or the requested entity exists or satisfies other pre-conditions.
RESOURCE_EXHAUSTED | 8 | Some resource has been exhausted, perhaps a per-user quota, or perhaps the entire file system is out of space.
FAILED_PRECONDITION | 9 | The operation was rejected because the system is not in a state required for the operation's execution. For example, the directory to be deleted is non-empty, an rmdir operation is applied to a non-directory, etc. Service implementors can use the following guidelines to decide between `FAILED_PRECONDITION`, `ABORTED`, and `UNAVAILABLE`: (a) Use `UNAVAILABLE` if the client can retry just the failing call. (b) Use `ABORTED` if the client should retry at a higher level (e.g., when a client-specified test-and-set fails, indicating the client should restart a read-modify-write sequence). (c) Use `FAILED_PRECONDITION` if the client should not retry until the system state has been explicitly fixed. E.g., if an "rmdir" fails because the directory is non-empty, `FAILED_PRECONDITION` should be returned since the client should not retry unless the files are deleted from the directory.
ABORTED | 10 | The operation was aborted, typically due to a concurrency issue such as a sequencer check failure or transaction abort. See the guidelines above for deciding between `FAILED_PRECONDITION`, `ABORTED`, and `UNAVAILABLE`.
OUT_OF_RANGE | 11 | The operation was attempted past the valid range. E.g., seeking or reading past end-of-file. Unlike `INVALID_ARGUMENT`, this error indicates a problem that may be fixed if the system state changes. For example, a 32-bit file system will generate `INVALID_ARGUMENT` if asked to read at an offset that is not in the range [0,2^32-1], but it will generate `OUT_OF_RANGE` if asked to read from an offset past the current file size. There is a fair bit of overlap between `FAILED_PRECONDITION` and `OUT_OF_RANGE`. We recommend using `OUT_OF_RANGE` (the more specific error) when it applies so that callers who are iterating through a space can easily look for an `OUT_OF_RANGE` error to detect when they are done.
UNIMPLEMENTED | 12 | The operation is not implemented or is not supported/enabled in this service.
INTERNAL | 13 | Internal errors. This means that some invariants expected by the underlying system have been broken. This error code is reserved for serious errors.
UNAVAILABLE | 14 | The service is currently unavailable. This is most likely a transient condition, which can be corrected by retrying with a backoff. Note that it is not always safe to retry non-idempotent operations.
DATA_LOSS | 15 | Unrecoverable data loss or corruption.
UNAUTHENTICATED | 16 | The request does not have valid authentication credentials for the operation.

https://grpc.github.io/grpc/core/md_doc_statuscodes.html

It's up to us to 'map' them to our business domain, or to establish conventions. For example, `NotFound`, can be used with the same meaning as in HTTP.