HTTP Filter Creation blog added

Signed-off-by: tanu70 <tauhedul@appscode.com>

content/post/envoy-hcm-filter/index.md

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+## HTTP Filter Creation
+In Envoy, we may use HTTP Connection Manager(HCM) for the HTTP based communication. In this filter, we can add some extra sub-filters, which
+are called HTTP filters.\
+In this blog, we'll write about how to create a new HCM filter.\
+\
+As there's no official documentation about this, we can't explain the design philosophy, we'll only mention what changes 
+should be done and what should be added to implement a new HCM filter and make it workable.
+
+- ### Proto
+  At first, we need to build the proto file for the new HCM filter. For this, create a new directory like `envoy/api/envoy/extensions/filters/http/new_hcm_filter/v3`
+  Here replace the `new_hcm_filter` with your filter name.\
+  Inside this directory, create a `BUILD` file and a `new_hcm_filter.proto` file. In the proto file we need to specify the
+  package name which should be something like `envoy.extensions.filters.http.new_hcm_filter.v3`, import the necessary files
+  and define necessary options for `go_package`, `java_package`, etc., and create a message something like `NewHCMFilter`.
+  We may follow any other http filter's proto file as a reference, for example we may follow the `router` filter.\
+  Inside the `NewHCMFilter` message we may add any required parameter we want to set from the Envoy configuration yaml.
+  Update the build file accordingly to support the imported file, follow other hcm filters for any help. Most of them has
+  similar kind of building dependency and structure.
+- ### Filter Implementation
+  Now, we need to implement the filter class and its config class maintaining the HCM filter protocol.\
+  Each HCM filter should inherit the `PassThroughFilter` class from the `source/extensions/filters/http/common/pass_through_filter.h` header file, 
+  if it's meant to be used as both read and write filter.\
+  In this filter we need to implement different functions from the `PassThroughFilter` class to have the functionalities of HCM
+  filter. Some of them are `decodeHeaders`, `decodeBody`, `encodeHeaders`, `encodeBody`, etc. We'll get the header of a request packet
+  going from the downstream to upstream in the `decodeHeaders` function, and the header of the response packet from the upstream to downstream
+  from the `encodeHeaders` function. Same for `decodeBody` and `encodeBody` functions.\
+  \
+  In the config file, we need to implement a FilterConfigFactory class that will be responsible to register the new filter in the filter chain.
+  Let's name this class as `NewHCMFilterConfigFactory` and inherit the `FactoryBase` class with specifying the template class name like
+  `FactoryBas<envoy::extensions::filters::http::elastic_search::v3::NewHCMFilter>`.\
+  Inside this class, implement the Constructor and override the `createFilterFactoryFromProtoTyped` function to create and register the filter class.
+  You may also override the `isTerminalFilterByProtoTyped` function and return `true` if the filter is a terminal filter. By default, it is false if you
+  don't implement. A terminal filter is a filter which must be at the end of the filter chain. Generally, a filter shouldn't be a terminal filter.\
+  For detailed understanding you may follow other HCM Filter's code as reference. Specially for the config part.
+- ### Adding References
+  To make it completely workable, we need to add the reference of the new filter in some places. Here are the files where we need to add the new filter:
+  1. Add the proto build package in the `envoy/api/BUILD` file. Inside this file add the `//envoy/extensions/filters/http/new_hcm_filter/v3:pkg` in the `v3_protos`.
+  2. In the `envoy/source/extensions/extensions_build_config.bzl` file's `Extensions` dictionary, add teh new hcm filter maintaining the format `"envoy.filters.http.elastic_search": "//source/extensions/filters/http/elastic_search_filter:elastic_search_config"`.
+  3. in the `source/extensions/extensions_metadata.yaml` file, insert the metadata for the new hcm filter maintaining the format:\
+    ```
+    envoy.filters.http.new_hcm_filter:
+     categories:
+       - envoy.filters.http
+     security_posture: robust_to_untrusted_downstream
+     status: stable
+     type_urls:
+       - envoy.extensions.filters.http.new_hcm_filter.v3.NewHCMFilter
+    ```
+  4. In the `source/extensions/filters/http/well_known_names.h` file, add the name for the new hcm filter inside the `HttpFilterNameValues` class as a const string.\
+    Example Format: `const std::string NewHCMFilter = "envoy.filters.http.new_hcm_filter"`.
+
+
+### Example config yaml
+```yaml
+http_filters:
+  - name: envoy.filters.http.new_hcm_filter
+    typed_config:
+      "@type": type.googleapis.com/envoy.extensions.filters.http.new_hcm_filter.v3.NewHCMFilter
+  - name: envoy.filters.http.router
+    typed_config:
+      "@type": type.googleapis.com/envoy.extensions.filters.http.router.v3.Router
+```
+
+
+### Conclusion
+As Envoy doesn't have any official documentation about creating a new HCM filter till now, the best way is to follow the other implemented 
+HCM filters as reference and maintaining their naming and structural formats.\
+In this blog we tried to sum up the mandatory changes that needs to be done to implement a new HCM filter and add it with the filter chain. This might change with time 
+if the envoy team changes their basic structure. But you might still get help from this.
+
+