Hooks

I/O hooks in DBL provide a powerful mechanism for intercepting and customizing file I/O operations, allowing developers to inject custom logic into the lifecycle of data access without altering existing code. This capability is especially useful in scenarios where you need to add functionality like logging, data validation, transformation, or encryption transparently to the I/O process. By leveraging I/O hooks, developers can gain finer control over how their applications interact with data sources, enhancing flexibility, maintainability, and the potential for sophisticated data handling strategies. This feature is invaluable both in developing new applications and enhancing existing ones, as it enables a seamless integration of custom I/O behaviors.

Using I/O hooks

To demonstrate the power and flexibility of I/O hooks in DBL, consider a scenario where you need to add logging for every read operation on a file. Instead of modifying every instance where a read operation occurs, you can create a custom IOHooks class that overrides the read_pre_operation_hook and read_post_operation_hook methods.

Implementing read operation ogging

First, define a class that extends the IOHooks class:

import Synergex.SynergyDE.IOHooks

namespace Example
    class LoggingIOHooks inherits IOHooks
        public method LoggingIOHooks
            ch, int
            parent(ch)
        proc
        endmethod

        protected method read_pre_operation_hook, void
            in flags, IOFlags
        proc
            ; Custom logic before a read operation
            Console.WriteLine("Pre-read hook: Starting read operation.")
        endmethod

        protected method read_post_operation_hook, void
            inout buffer, a
            in flags, IOFlags
            inout error, int
        proc
            ; Custom logic after a read operation
            Console.WriteLine("Post-read hook: Read operation completed.")
        endmethod
    endclass
endnamespace

In this class, read_pre_operation_hook is executed before any read operation, and read_post_operation_hook is executed after. Both methods log messages to the console.

To use this custom IOHooks class, create an instance and associate it with a file channel:

main
record
    channel, int
record rec
    somedata, a100
proc
    open(channel=0, i:i, "mydatafile.ism")
    new BufferedLoggingIOHooks(channel)
    
    ; Perform read operations as usual
    reads(channel, rec)
    ; The hooks will automatically log messages before and after the read

    close channel
    channel = 0
end

When the READS statement is executed, the pre- and post-read hooks are automatically called, adding logging without changing the original read operation.

Global I/O hooks for easier adoption

For an application that wasn’t designed with a single routine for all file opens, consider using global I/O hooks. Inside the global hook, you get enough information to decide if you want to hook the file and potentially determine which hook if you have multiple. To implement global hooks, you need to define them in the SYN_GLOBALHOOKS_OPEN routine:

subroutine SYN_GLOBALHOOKS_OPEN
    in channel, n
    in modes, OPModes
    in filename, a 
proc
    if(filename == "mydatafile") then
        new LoggingIOHooks(channel)
    else
        nop ; Do nothing, it's not our file
endsubroutine

In this example, after each file is opened, the global hook activates, and the logging functionality is applied to all read operations across the application.

Pitfalls of I/O hooks

While powerful, I/O hooks in DBL execute synchronously with I/O operations. This synchronous execution means that the custom logic in the hooks runs in the same thread and directly affects the performance and behavior of the I/O operation. Misusing I/O hooks can lead to several detrimental effects on an application, such as performance degradation, data inconsistency, or unintended side-effects.

Performance impact

Consider a scenario where you have implemented a read_post_operation_hook to perform complex data processing or external API calls:

namespace Example
    class HeavyProcessingIOHooks inherits IOHooks
        public method HeavyProcessingIOHooks
            ch, int
            parent(ch)
        proc

        endmethod

        protected method read_post_operation_hook, void
            inout buffer, a
            in flags, IOFlags
            inout error, int
        proc
            ; Complex data processing
            ; or time-consuming external API call
            ;PerformComplexDataProcessing(buffer)
        endmethod
    endclass
endnamespace

Using this I/O hook, every read operation on the file waits for the PerformComplexDataProcessing method to complete. This can significantly slow down the overall performance of your application, especially if this file contains a lot of records that need to be read.

Mitigating performance impacts in I/O hooks

The use of I/O hooks, particularly for recording operations, can have a noticeable impact on application performance. Since I/O hooks methods are invoked synchronously with I/O operations, any additional processing time they introduce directly affects the overall I/O performance. To mitigate these impacts, it’s crucial to optimize the hook methods and consider asynchronous or lightweight logging strategies.

Use lightweight logging Opt for minimalistic logging formats and avoid complex string operations. For instance, record only essential details such as operation type, timestamp, and identifiers.

Buffered logging Instead of writing logs to a file or external system directly within the hook, consider buffering these logs in memory and writing them in batches. If your scenario allows for the potential buffering delay, this reduces the I/O overhead within the hook.

import System.Collections
import Synergex.SynergyDE.IOExtensions

namespace Example
  class BufferedLoggingIOHooks inherits IOHooks
      private const BUFFER_THRESHOLD, int, 100
      protected logBuffer, @ArrayList
      ; Other hook methods

      public method BufferedLoggingIOHooks
          ch, int
          parent(ch)
      proc

      endmethod

      protected method write_post_operation_hook, void
          inout buffer, a
          in flags, IOFlags
          inout error, int
      proc
          ; Append log to buffer
          logBuffer.Add(BuildLogEntry(buffer))
          ; Periodically flush the buffer
          if logBuffer.Count >= BUFFER_THRESHOLD then
              FlushLogBuffer()
      endmethod

      private method BuildLogEntry, @a
          buffer, a
      proc
          ;;format the log entry here and return it
          mreturn (@a)buffer
      endmethod

      private method FlushLogBuffer, void
      proc
          ;;write out all of the entries from logBuffer and clear it
      endmethod

  endclass
endnamespace

Offload to external processes Consider delegating the heavy lifting or external network operations to a separate, asynchronous process or thread. This way, the main I/O operations aren’t blocked by logging activities.

Selective hooking Be selective about which operations to hook. Not all I/O operations might need logging or additional processing. Consider hooking only the operations that require it.

Implement caching If your hooks involve data lookup or processing that can be cached, implement a caching mechanism to avoid redundant operations. For example, cache the results of database lookups or complex calculations.

Keep it simple The logic within each hook method should be as simple and efficient as possible. Avoid unnecessary computations or complex logic within the hooks.