What is middleware exactly?

It is just a piece of software or a tool on which your application executes and rapplication capabilities with respect to high availability,scalability,integrating with other softwares or systems without you bothering about your application level code changes .

For example : The operating system on which your application runs requires an I.P change , you do not have to worry about it in your code , it is the middleware stack on which you can simple update the configuration.

Example 2 : You experience problems with your runtime memory allocation and feel that the your application usage has increased , you do not have to much about it unless you have a bug or bottleneck in your code , it is easily achievable by tuning middleware software configuration on which your application runs.

Example 3 : You have multiple disparate software and you need them to talk to each other or send data in a common format which is understandable by all the systems then this is where middleware systems comes handy.

Hope the information provided helps.

There is a common definition in web application development which is (and I'm making this wording up but it seems to fit): ***A component which is designed to modify an HTTP request and/or response but does not (usually) serve the response in its entirety, designed to be chained together to form a pipeline of behavioral changes during request processing.***

Examples of tasks that are commonly implemented by middleware:

- Gzip response compression
- HTTP authentication
- Request logging

The key point here is that none of these is fully responsible for responding to the client. Instead each changes the behavior in some way as part of the pipeline, leaving the actual response to come from something later in the sequence (pipeline).

Usually, the middlewares are run before some sort of "router", which examines the request (often the path) and calls the appropriate code to generate the response.

Personally, I hate the term "middleware" for its genericity but it is in common use.

[Here is an additional explanation specifically applicable to Ruby on Rails.][1]

[1]:

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`Middleware` stands between web applications and web services that natively can't communicate and often are written in different languages/frameworks.

One such example is `OWIN middleware for .NET` environment, before owin people were forced to host web apps in a microsoft hosting software called `IIS`. After owin was developed, it has added capacity to host both in IIS and self host, in IIS was just added support for Owin which acted as an interface. Also it become possible to `host .NET web apps on Linux via Mono`, which again added support for Owin.

It also added capacity to create `Single Page Applications`, Owin handling `Http` request/response context, so on top of owin you can add `authentication/authorization` logic via OAuth2 for example, you can configure middleware to register a class which contains logic of user authentification (for ex. OAuth2 implementation) or class which contains logic of how to manage http request/response messages, that way you can make one application communicate with other applications/services via different data format (like json, xml, etc if you are targeting web).

it is a software layer between the operating system
and applications on each side of a distributed computing system in a network. In fact it connects heterogeneous network and software systems.

If I am not wrong, in **software application framework**, based on the **context**, you can consider `middleware` for the following **roles** that can be combined in order to perform certain activities in between the user `request` and the application `response`.

- Adapter
- Sanitizer
- Validator

Lets say your company makes 4 different products, your client has another 3 different products from another 3 different companies.

Someday the client thought, why don't we integrate all our systems into one huge system. Ten minutes later their IT department said that will take 2 years.

You (the wise developer) said, why don't we just integrate all the different systems and make them work together? The client manager staring at you... You continued, we will use a Middleware, we will study the Inputs/Outputs of all different systems, the resources they use and then choose an appropriate Middleware framework.

*Still explaining to the non tech manager*
With Middleware framework in the middle, the first system will produce X stuff, the system Y and Z would consume those outputs and so on.

I would say middleware is like a middleman.
Helps to communicate between two or more different type of guys.
Its just like a translator.
Same way in our computer world or systems, there are variety of systems communicating with each other like frontend with backend for some data from the database and they want to understand each other by using suitable and compatible information. That's where middleware comes into play. It takes information from one system and modifies it and helps other system to understand that information.