etag-conc-example

# 1) POST a new Event to the server
curl -i -X POST -H "Content-Type:application/json" \ 
                -d '{"name":"Basilica Block Party", "description":"Test description"}' \
                http://localhost:8080/events


# 2) Notice that an ETag header is populated with the contents of the version field 
#    (starting at 0) 
HTTP/1.1 201 Created
Server: Apache-Coyote/1.1
ETag: "0"
Last-Modified: ...
Location: http://localhost:8080/events/555557f6d4c6560c6f87f186


# 3) Update the description of the event with a PATCH. The If-Match header matches 
#    the current resource ETag so this update will work.
curl --request PATCH -i -H "Content-Type:application/json" -H "If-Match:0" \
                        -d '{"description":"Updated description"}' \
                        http://localhost:8080/events/55555bc2d4c6f98f355b003e

# Update succeeds and the ETag is incremented
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
ETag: "1"
Last-Modified: ...


# 4) Try another update, but use an out-of-date If-Match value of 0. This simulates the 
#    scenario where the resource was requested, but another client or user changed the
#    resource before this PATCH could be applied.
curl --request PATCH -i -H "Content-Type:application/json" \ 
                        -H "If-Match:0" \ 
                        -d '{"description":"This update should NOT work"}' \
                        http://localhost:8080/events/55555bc2d4c6f98f355b003e

# The server returns a 412 to notify you the resource changed before the PATCH 
# and a fresh GET should be executed so you don't overwrite the changes of another client.
HTTP/1.1 412 Precondition Failed


# 5) At this point the ETag for this resource is "1", so update the If-Match to that. 
curl --request PATCH -i -H "Content-Type:application/json" \
                        -H "If-Match:1" \
                        -d '{"description":"This update SHOULD work"}' \
                        http://localhost:8080/events/55555c2d4c6f98f355b003e

# This update worked since the If-Match value matched the current ETag. It is incremented to 2.
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
ETag: "2"
Last-Modified: ...