Currently in Hyperledger Fabric 1.0 there is a central membership service. I want a way to make it decentralized so that atlas 50% of the members have to agree for a new member to join the network. How can I achieve this?
The idea is basically put the membership logic in chain code and let member service fetch data from chain code at the time of enrollment. But how to enforce this, I mean how do we know that membership service is actually reading from blockchain and not cheating.
This is actually natively support by Hyperledger Fabric, and the behavior you describe is actually the default for channel membership changes.
Each channel begins life with a genesis block. The contents of this genesis block define the channel members, as well as policies for which users from these organizations are authorized to perform different functions on the blockchain. For instance, some users may be allowed to submit transactions, but not read the whole blockchain, while others could do both.
To change the channel membership, you submit a channel reconfiguration transaction. This transaction specifies the new membership, and must include enough signatures to authorize this modification. By default, this is signatures from the admins of a majority of the organizations.
The policy framework is actually quite powerful, and with a little knowledge, you can define even more powerful rules. For instance, you could require that OrgA and 3/10 other organizations sign off to change membership. Or, you could require that all but one Org agree to make any membership change, or an infinite number of other permutations.
Some links you might find helpful:
http://hyperledger-fabric.readthedocs.io/en/latest/configtxgen.html
http://hyperledger-fabric.readthedocs.io/en/latest/policies.html
http://hyperledger-fabric.readthedocs.io/en/latest/configtx.html
The documentation and tools around reconfiguration are a little lacking at the time of this writing. The most useful place you can probably look is:
https://github.com/hyperledger/fabric/tree/release/examples/configtxupdate
There are two protobuf structures you must familiarize yourself with, the common.ConfigUpdate, and the common.Config. Channels are created by submitting a signed config update to the ordering service, which generates a corresponding config embedded in the genesis block.
The policy which governs membership changes for a channel is specified as the mod_policy field of the Application group, which is a subgroup of the Channel group. This field defaults to Admins, which refers to the policy definition Admins within the Application group. By default, this policy is set to MAJORITY of the Admins policies for the organization groups defined under the Application group.
So, to modify this policy before creating your channel, you would decode the configtx to JSON using the configtxlator tool, make your modifications, and then encode it back using the configtxlator tool once again. Submitting this new transaction will create the channel with the policy you specified.
If you wish to modify membership after the fact, the process is similar. Retrieve the current channel configuration, decode and modify it, then use configtxlator to compute a config update structure which represents your change. Gather signatures via peer channel signconfigtx then submit it to modify your channel's configuration.
This process is obviously all a bit manual at the moment, but in the future, common tasks should be automated by the SDKs and the tooling should improve as well.
Note: configtxlator is a REST service so that it can be accessed conveniently from inside your SDK application, independent of language.
As a quick addendum. You asked how you can be sure that no one is 'cheating' and not really getting the required signatures. This is also built into the system. All changes to the channel configuration are validated not only by the ordering network, but by all peers in the system. If a configuration arrives which cannot be validated, then all nodes in the network will notices, and will halt usage of that channel until corrective administrative action is taken.
For decentralised membership, that is not dependent on a centralized CA, take a look at Blockstack.
