microsoft/FASTER · error · FasterException

Cannot use empty string as session name

Error message

Cannot use empty string as session name

What it means

FASTERClientSession.InternalNewSession treats an empty string session name as invalid: null means 'auto-generate a name', so "" is ambiguous and would collide with FASTER's naming/recovery conventions. It throws FasterException instead of registering a session with an empty name.

Solutions

1. Pass null instead of "" to let FASTER auto-generate a unique session name.
2. Provide a non-empty descriptive name, e.g. $"session-{Guid.NewGuid()}" or a business identifier.
3. Validate sessionName with string.IsNullOrWhiteSpace before calling and substitute a default.

Example fix

// before
var session = fasterKV.NewSession(functions, sessionName: ""); // throws

// after
var session = fasterKV.NewSession(functions, sessionName: null); // auto-generated name
// or
var name = string.IsNullOrWhiteSpace(configuredName) ? $"session-{Guid.NewGuid()}" : configuredName;

string sessionName = string.IsNullOrEmpty(configuredName) ? null : configuredName; // null = auto-generate
var session = fasterKV.NewSession(functions, sessionName);

string NormalizeSessionName(string name) => string.IsNullOrWhiteSpace(name) ? null : name;

try
{
    var session = clientSession.NewSession<MyFunctions>(sessionName);
}
catch (FasterException ex) when (ex.Message.Contains("empty string"))
{
    var session = clientSession.NewSession<MyFunctions>(); // auto-generated name
}

When it happens

Trigger: Passing sessionName: "" to the session-creating API that funnels into InternalNewSession (e.g. NewSession with an explicitly empty string name).

Common situations: Session names read from config/environment that default to empty strings; string interpolation producing "" when an ID part is missing.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

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AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/827aef3dd179059e. Report an issue: GitHub.

Appendix: source

Thrown at cs/src/core/ClientSession/FASTERClientSession.cs:167

        /// <param name="sessionName">Name of session (optional)</param>
        /// <param name="sessionVariableLengthStructSettings">Session-specific variable-length struct settings</param>
        /// <param name="readCopyOptions"><see cref="ReadCopyOptions"/> for this session; override those specified at FasterKV level, and may be overridden on individual Read operations</param>
        /// <returns>Session instance</returns>
        internal ClientSession<Key, Value, Input, Output, Context, Functions> NewSession<Input, Output, Context, Functions>(Functions functions, string sessionName = null,
                SessionVariableLengthStructSettings<Value, Input> sessionVariableLengthStructSettings = null, ReadCopyOptions readCopyOptions = default)
            where Functions : IFunctions<Key, Value, Input, Output, Context>
            => InternalNewSession<Input, Output, Context, Functions, ClientSession<Key, Value, Input, Output, Context, Functions>>(functions, sessionName,
                            ctx => new ClientSession<Key, Value, Input, Output, Context, Functions>(this, ctx, functions, sessionVariableLengthStructSettings), readCopyOptions);

        private TSession InternalNewSession<Input, Output, Context, Functions, TSession>(Functions functions, string sessionName,
                                                                            Func<FasterExecutionContext<Input, Output, Context>, TSession> sessionCreator, ReadCopyOptions readCopyOptions)
            where TSession : IClientSession
        {
            if (functions == null)
                throw new ArgumentNullException(nameof(functions));

            if (sessionName == "")
                throw new FasterException("Cannot use empty string as session name");

            if (sessionName != null && _recoveredSessionNameMap != null && _recoveredSessionNameMap.ContainsKey(sessionName))
                throw new FasterException($"Session named {sessionName} already exists in recovery info, use RecoverSession to resume it");

            int sessionID = Interlocked.Increment(ref maxSessionID);
            var ctx = new FasterExecutionContext<Input, Output, Context>();
            InitContext(ctx, sessionID, sessionName);
            ctx.MergeReadCopyOptions(this.ReadCopyOptions, readCopyOptions);
            var prevCtx = new FasterExecutionContext<Input, Output, Context>();
            InitContext(prevCtx, sessionID, sessionName);
            prevCtx.version--;
            prevCtx.ReadCopyOptions = ctx.ReadCopyOptions;

            ctx.prevCtx = prevCtx;

            if (_activeSessions == null)
                Interlocked.CompareExchange(ref _activeSessions, new Dictionary<int, SessionInfo>(), null);

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